OK, I'm back from my test ride. A lot of fun! Ride and a couple pictures here on Strava
https://www.strava.com/activities/794194284To answer the question about a fork for the CS-496 frame. You have 2 main options. A 120 mm Reba is a good and economical choice. I plan on being less reasonable and pick a Pike, 140 mm. It is a lot more rigid then the Reba. The one on the Trek today was indeed really nice (but "only" 120 mm).
Then, I think I just can't be unbiased about my test of the Stache
. Overall, I really prefer my bike. Faster climbing (less weight in the wheels, as well as total weight, almost 2 kg lighter). Gyroscopic effect is very strong at high speed. It's a very good bike, but not what I want. I can't help to push and beat my buddies or my own past records climbing steep and technical hills, 27.5+ is just better for that. Then, it's true that going down, the Trek is very stable and smooths the trails better then my bike but still not close to a full-sus. To be honest, I had the feeling on the Stache to get a full-sus with suspension locked.
So, for what I do a 27.5+ is what I need. But if I want to smooth the trail more I'd go Full-sus (27.5+ still) rather then 29+. I think it really depends on where and how you ride.