I think the MX is also a bigger chassis?
I don't run my 25 series (call it identical to the 20 for size) in the woods. I've been over rough ground for sure, plenty of mud, and certainly plenty of snow (including the one and only time I managed to bury it in snow to the frame). I've never damaged anything underneath mine, but again, I'm not running it in heavy woods. I've been through tree rows, small stands of trees and some woods mowing and clearing land, but I'm always pretty careful to watch all my angles and overhead limbs (I have a cab). I do a lot of field mowing in the summers, most of the time (ok, all of the time now) it's on someone else's property, so I'm less familiar with what is lurking under the grass. So I intentionally leave my FEL and bucket (or sometimes forks) on and keep the bucket low in front of me, say a foot or slightly more off the ground. This acts as a "feeler" if I'm running in heavy grass on an unfamiliar field that someone "forgot" to tell me had a rock pile, or tree stump, or strings of old fence wrapped up in fence posts, or an old car frame sitting in a corner, etc.
Nothing under the Branson is going to be armor plated, but I've certainly been over a ton of brush, sticks, heavy tall grass, etc with mine. I know a few members here have indeed damaged things underneath theirs, I think one member punched a hole in a filter, or tore it off, something like that. Also a couple of members (various brands) have speared their radiators through their grills by pushing through heavy woods. Some expanded metal across the front brush guard will help that. I haven't done that yet, but I probably should.
If you're looking for a dedicated heavy, thick woods tractor, maybe the MX is worth the look and the extra $?