The Xc-marathon can be considered as the step preceding the all mountain since these bikes are already addressed to the hikers in the long course, which roll on technical courses (but not that) and rather prefer the long loops from where the research a good comfort.
For this kind of practice, there are 120 mm in 27.5 "and 100 mm in 29". In terms of comfort, these bikes are very close despite the clearance gap due to large wheels. These two families of bikes will serve the same purpose, they are targeting the same practitioners.
From a point of view of the driving position, these bikes are often a little less radical than the 100 mm, with a shorter stem, a handlebar often semi-raised and brakes 160 or 180 mm in diameter. Slightly less climbing than the 100 mm, they are often much easier to descend.