power on bike - one source of truth and you can use power everywhere 
Do you place more value on training with power indoors or outside?
I used TR for 3 years with a dumb trainer and power meter; i bought a Neo 2 this summer and I wish I had made the change sooner.
The overall experience of a smart trainer is just…better. All types of rides are better—no more hunting for the “right” gear/cadence/resistance/pressure with ever workout. Turn it on and pedal. It’s that’s simple.
If you hate riding the trainer and put more weight on your outdoor training, then you may look at optimizing the on-bike power meters.
Sounds like you think they’re about the same cost and that the PM on the trainer bike would have no value other than being on the trainer. In that case, I’d get the smart trainer.
The nice thing with Erg mode is that if it’s not working for you, you can turn it off.
If you can mount your outdoor bike to your trainer, ride it once to calibrate that PM vs the trainer, then be done with that problem
I don’t know anything about P2M. Would installing one in two bikes give the same power measurement in terms of accuracy?
Or would you be better off with Assiomas or another pedal based system you can change between bikes?
Also- power meter first.
you can use a PM inside and out, seems like a no brainer
I would go power meter, but I would go pedals so you can move them from bike to bike … then you can use them to indoor train or outdoor ride … one source of the truth
For me, dumb trainer and 2 powermeters. You absolutely do not need a smart trainer to execute TR workouts successfully. I have a KK Road trainer and it’s more than good enough.
PM first for me.
However, if you’re options are two powermeters, or one PM and a top-end smart trainer, I wonder if your budget wouldn’t also stretch to 2 PMs and a cheap smart trainer. Mine cost £200.
First things first - you have what I consider to be a false assumption in your original post. Having two separate power meters on two separate bikes, even if they are the same brand and model, does not mean they will 100% align. Therefore, having two power2max power meters (one per bike) does not guarantee any more consistency than having a power meter on your outdoor bike and a smart trainer as your power source indoors
If you were considering buying a single power meter and using that as your source of truth indoors and outdoors that would lead me to recommend that option.
However, since both options involve introducing two sources of power (either a trainer + power meter or two separate power meters) I would recommend you buy the trainer and the power meter.
With this option you can put your outdoor bike (with power meter) onto the trainer and reconcile the differences between the two and thus have knowledge about the discrepancy in the readings (hopefully minimal). You can then put your indoor bike back on the trainer and have a solid understanding of the gaps between the two sources for all future rides
With your other option (two power2max units) it will be impossible to compare them directly - so you’d never truly know what, if any, discrepancy exists between your indoor and outdoor training.
So…either go with a third option (as others have suggested - an easily swappable power meter such as one of the many pedal based systems) or go with option 2 and buy a direct drive smart trainer and a power meter
Note: I’m referencing potential inconsistencies in power2max because that’s the brand you mentioned - I have one of these and absolutely love it - I’m not throwing shade at them in particular, any two power meters from any manufacturer can have this problem
IMHO erg mode as a training tool is overrated. Its a nice addition (for certain types of intervals - on others it truly sucks). But, it does not allow you to do anything you cannot do with just a power meter. At best, it just lets you do some types of riding with less mental focus. Nice, but not necessary.
On the other hand, having power on every single ride, indoors and out, is a true game changer. Not only can you do structured training both indoors and out, you can then track other very useful metrics to guide your overall training.
So, never ever get a smart trainer instead of a power meter if you are truly serious about training.
The only exception to that recommendation would be if you intend on spending a lot of time on Zwift and are willing to sacrifice outside to get a better experience. Zwift really requires a smart trainer to get the full experience.
I have four power meters and two smart trainers.
My vote is for option #2.
I understand you want your indoor and outdoor power readings to match exactly. IMHO you’re overvaluing the importance of that. With a quality PM and a quality smart trainer, they will be close, and you can map out the discrepancies by putting your outdoor bike on the trainer and recording the PM separately on a head unit.
I’ve both. I do some workouts on hybrid rollers with power meter (in work)*, but I wouldn’t be without the smart trainer. I definitely prefer erg - just to take gearing out of it. Also, for less intense stuff, I don’t really need to be fully engaged with erg, where as as soon as I lose focus on the hybrid rollers, I miss power targets, so even endurance workouts are more mentally draining.
I haven’t tried outdoor workouts yet, as by the time they came to the bolt it was too late for me this year, so notwithstanding that I find the power on the road interesting data, and handy for calorie burn, but to be honest that’s it. My FTP doesn’t match anyway, as I’m able to be above my indoor FTP for over an hour in race situations!
*Only in SSB1, not sure how this is going to work once I get to VO2 Max in SSB2. I’ll see how that goes, but with my current training regime working schedule wise, I’m really waiting for @dcrainmaker and @gpl to have the full look at the Infliiiiiiiiiiiiiiiiight ( 
)
I went the smart trainer route. Except i have no power meter.
I’m hoping that training with the smart trainer will make more aware enough of what endurance vs recovery vs sweet spot feels like on race day.
But i know that when i start doing half iron man and training for ironman, the power meter is going to be a big part of my training. Thankfully my setup is scalable, but i just don’t see the necessity of having one when i should just focus on getting better/faster.
Once i reach a few years of xp, the specificity of the watts will be more important, whereas now, i can accept a + or - 5% bias and RPE or HR training when outside
Can almost guarantee once you’ve trained with power, you always want to see power
Smart trainers are a nice novelty, but power meters are really useful for training indoors and out.
Always power meter over smart trainer if I could only have one. ERG mode is highly overrated and doesn’t really do anything that will help make you faster.
The only exception I can think would be if you only ride inside except for race days. Then I’d go with a smart trainer. I’d you like to train outside though, you’re definitely gonna want power.
Thanks for the replies. It is helping me decide on what I’m going to do.
When I posted last night I was leaning towards the two power meters, mostly because of cost, but after reading some of these posts I’ll probably go the smart trainer route.
@Quadzilla_Jr made a great point. I place more value on training with power indoors. All of my structured training is indoors and it makes more sense to have the smart trainer. When I’m riding outdoors, it’s usually racing, riding with friends, or unstructured solo riding. So a PM is not really needed.
@trpnhntr’s point was something I considered but then I also realized that my indoor and outdoor FTP might not match anyway so it doesn’t matter that I have the same model PM on both bikes.
I considered pedals but I have Time ATAC mtb pedals on all three of my bikes and I don’t want to change that. I also considered a powertab hub, but I don’t want to have to keep moving a wheel from bike to bike.
The first thing to note is that your indoor FTP will NOT be the same as your outdoor FTP. So, it doesn’t matter if you’re using two completely different power meters. This is mainly due to efficiency in cooling your body.
The second thing that I would consider is how much do you like your current trainer setup? The cheaper smart trainers can be had for about $100 more than a crank based power meter (even less if you’re looking at more expensive cranks).
If you’re looking at maximizing the functionality for best price, look for New Old Stock first gen smart trainers such as the Cycleops Magnus. There’s are still plenty out there for sale brand new in the box even though the M2 has been out for over a year. There are also some really good deals on Ebay and Aliexpress for some power meter brands that haven’t made it to the states, but are very popular in China. Most of them are only for Shimano cranksets though. Personally, I’ve been using an xCadey power meter on an Ultegra crankset for just over a year. It’s still working great. They can be purchased online for about $300.
If you truly only want to make a single purchase, I’d go with the power meter for your outdoor bike. If all you’re doing on the trainer is training (not eSport racing) then virtual power will get the job done. The fact that your virtual power FTP won’t translate to your outdoor bike doesn’t really matter. You’ll want to test your outdoor FTP anyway as it is most likely going to be 10 to 20 watts different from your indoor FTP even if you’re using the same power meter indoor and outdoors.
Stay away from the powertap hub. It costs almost as much as a crankbased power meter and then you can’t upgrade wheels. You have to factor in the cost of building it into a wheel and by then it’s not much of a deal. Also, I recently started using my cross bike as a road bike as well (my road bike is endurance geometry and I realized I like the race geo better) and so now I have no power on my bike because the power is in the cross wheel hub hanging on the wall.
I think you’ve nailed it in terms of what your intentions are both indoor and outdoors as I’ve had the same decision to face. Weather and area is also a factor for some and with the cold rain or icy days, it made sense for me to lean to a smart trainer to get through winter. Good luck with your training and decision!