Messicks have a Grand L to MX comparison:
Kubota Grand L Series VS. Kubota MX Series - YouTube. There's also a Grand L to standard L comparison:
Kubota Standard L Series VS. Grand L Series - YouTube. Presumably between those two the answer lies?
My guess is the L is a lighter tractor, suitable for people who want lots of HP (probably to drive mowers and the like) but don't want lots of weight (they're not pulling heavy things, and they don't want ruts in their lawn). The MX is more suitable for people who want a farm tractor - pulling a plow, lifting heavy stuff, pulling heavy trailers. In those situations you want weight. I think it also lifts a bit higher and a bit heavier? Basically you're comparing the highest HP version of the smaller tractor to the lowest HP version of the next model up. It's the overlap point in the lineup. So the question is whether you want the "more HP" version of a smaller tractor, or the cheapest version of the larger tractor.