Depends on the trails. If all you do is ride groomed trails, then you may not like the reduced top speed compared to the SXR. The MM has 8 tooth drivers and lower gearing, so it tops out around 85 or so. The 2" track won't hold up too well on hard trails if ridden aggresively. Like Turk said, you have to jump off into the soft stuff once in a while to keep the hifax and track happy.
On the other hand, if the trails are rough, the longer track bridges the bumps better and actually feels better to me. I won't ever own another 121 machine. 136 and longer is more comfortable and 136-144 isn't that much different in handling from the 121. If you're lucky enough to be the first on the trails after a storm, and before the groomer, the longer track is worth it's hard trail shortcomings.
The only other issue I have with the MM on the trails is that I wish the front had more travel. 6" is OK, but more travel would sure be nice.
Just my LONG $.02