Absolutely regret buying a single sided. While the power variance is not so bad you can’t train with it I think a lot of problems people run in to is comparing one bike vs another or one bike with a power meter vs a trainer bike with a smart trainer, etc.
I have consistently about a 2.5% lower power output from my left leg so this adds up to a 5% lower number when it is doubled. This can sometimes be even worse when I get tired or I’m doing V02 intervals. The issue that makes it even worse is I have single sided on my gravel and mountain bikes but spider based on a CX bike and I also have a Kickr. With this many power meters I realized what the issue was and it is really annoying to perform testing on a Kickr with a different power meter only to find out your numbers are way off on the gravel bike which is what most of my intervals are on.
My 2 cents from having power meters on my bikes since the days of the hard wired CycleOps rear hubs: The best power meter platform in my opinion is pedal based. They measure a full power picture, they can be swapped between bikes, they don’t lock you in to a certain crankset or chainring combination, and are easily taken when traveling.
I have spent far too much money getting say a Stages left crank arm to match my Dura Ace 9000 crankset, then selling the bike a few years later and getting a bike with SRAM Red. A ton of wasted money doing things like that. I highly recommend if you have a road bike to go with power meter pedals and look for a quality used set and you can use them on all your bikes except mountain. I also expect in the near future we are going to see multiple gravel SPD based options since the patent for SPD cleats just expired at the end of 2019. Once this occurs I will probably swap over to one of those options for my gravel/road bike and keep whatever PM I have on my MTB at the time as I don’t do many intervals on that bike.
Bottom line…go dual and save yourself a bunch of headache.