Road rider thinking about doing one of the Mountain Races next year. To turn the dream into reality I’ll need to pick up a new bike some time this summer. Looking at all the various XC options, but I have zero MTB experience to use as a basis for making good choices. I’m expecting a 6 to 8 day event. I think almost all the choices out there can handle what I’ll need for storage, but I’m not sure how to choose between hard tail and full suspension and mech vs electric. Any thoughts or advice to offer? Anything to make sure I get or definitely avoid? Course will be mostly gravel / dirts roads but with some pavement, single track, and hike-a-bike mixed in for extra fun. The folks at bikepacking.com publish bike / kit photo galleries, so I’ve looking through them. Unfortunately they’re all pre-ride, so they don’t give any insight into what worked well and what didn’t.
I’d go a hard tail mtb at a minimum, with aero bars you feel very comfortable on. May strongly consider full suspension.
There’s tons of good YouTube videos of personnel riding the Atlas. Think I’ve watched a few, particularly of Josh Reid has done it (he makes good videos). That’d be a good chance to see what more people are on as well.
After years of riding a hardtail, I don’t know if I’ll ever own one again.
Full suspension bikes offer so much more grip and comfort that I just don’t have the desire to ride a hardtail anymore. They are still great bikes, but I think I’ve moved on. 
If I don’t need rear suspension, I’d probably just go with a modern gravel bike with 2" tires.
A 120/120mm or 120/130mm bike will be somewhat future-proof and likely get you everywhere you need to go for quite a while.
I’m a fan of the Specialized Epic 8 (I’m sure the 9 is great also), and the Trek Top Fuel. The new Giant Antem, Scott Spark, and Caynon Lux both look great as well.
Have fun!
I think the comfort of a full suspension bike is hard to beat. What about something really efficient like the Trek Supercaliber? I’m not sure you need 120mm travel, just something to take the sting out of the trail … Also mechanical drivetrain is surely the way to go.
I think if you’re going to go hardtail you want either steel, titanium, or aluminum, not carbon. Carbon hardtails are harsh, as are aluminum, but aluminum is cheap at least. Ti and steel are more comfortable. Ti is expensive of course. Not familiar with these courses but generally the weight and power transfer of a hardtail is nice for long, not too technical courses. The more technical the course, the more your back, arms, and legs are going to feel the fatigue from a hardtail over long distances. These courses seem non-technical. If I was picking just for these events, I would do a hardtail with 120mm of suspension, aiming for a bike weight of 20-22 lbs. Cost would be like $3-$5k USD new, perhaps a little more for Ti. You also don’t really want to be strapping bags onto a carbon frame. For full suspension, your bike is going to weigh 24-26ish lbs probably, and cost like $5-$10k USD. You will lose in-frame storage on the full suspension.
If I was you I would look into the used marketplace for a lightweight steel, Ti, or quality aluminum frame or full build.
If it was me, I would do Ti 29 hardtail, but I also like to MTB so the investment in Ti makes sense.
Oh also I would 100% do mechanical drivetrain. Shimano XT 11 or 12 speed. Probably 12 to give you a granny gear - 10-51T cassette. Might be worth looking into options for a 30T front chainring. I believe the stock smallest size is 32T but there may be aftermarket adapters or cranks. Lots of people find 32-34 the speetspot for regular riding so I suspect when you have loaded frame you might want 30, depending on your fitness of course and the elevation profiles. For the derailleur - the older clutch version, not the newest version they are releasing this summer which appears to be clutchless. You don’t want to worry about charging batteries during this event, unless this is already something you’re used to from your road bike. Mechanical is lighter than electrical too, for a given quality level, and way cheaper.
BTW I did a quick look into the bikepacking.com articles and saw one of the guys riding a Fairlight Holt. I would 100% buy that bike if it was a good size for me, which it is not (i’m very short). The bikepacking world raves about Fairlight frames, particularly the Secan.