Hmmm...good question.....brain fart I guess. Got my numbers wrong
Just going by the specs, the MX5100 and L5240 are darn near the same tractor. An inch or two difference in some dimensions, about 150 lbs difference in weight (the L is heavier), same tires, same hydraulic capacity, same turbocharged engine (I think ), same turn radius, etc. The newer Grand L's have a souped-up, 2 piston 3 point lift system that lifts quite a bit more than the one on the MX, and the Grand L's have the HST+ and the electronic instrument cluster not offered on the MX.
I looked at an MX at a farm auction recently and thought it looked very similar to my older Lxx10, at least mechanically. No electronics, new HST, etc., but still a very nice, hard-working tractor. Considering the MX5100 lists for $5k less than the L5240, it looks to be an excellent value.