A midsize GL will be more maneuverable than the MX and while a smaller tractor will go everywhere a MX will, the converse cannot be said. Generally speaking, a "smaller" tractor will do anything a larger one, but a larger one simply won't fit in some places a smaller one will.
As is often said, "Bigger isn't always better, sometimes it is just bigger."
Not only will a larger tractor sometimes not fit where you are trying to do work, but sometimes they are just harder to use for certain tasks. After trying to do precise loader work or finish grading with our M9540, I'm actually relieved to get back on my little
B3200 to do the same tasks. Granted this is a much more extreme size difference than a GL vs MX, but the principles are still the same. With the larger tractors you are just so far away from the attachments that visibility is poor at best. With the loader you just have to watch the level indicator and have a 'feel' for what the bucket is doing and where it is. Using pallet forks is a guessing game as well on a large tractor.
Really it's all about sizing the tractor to the tasks, there are still a lot of things the big Kubota can do that mine can't. Like loading our dump truck, stacking round bales three rows high, lifting materials up to the roof of a building, etc. Those are all cases where there is a specific need for that size tractor.
Just to throw something else into the decision, if you want a small tractor with lots of features and capacity, the
B26 Industrial tractor/loader is something else to consider. It has most of the features of the grand L's, a quite substantial loader for it's size, smooth running, nice 3pt hitch and PTO. But it's also fairly compact and very easy to work with. The price on the
B26 with a backhoe is up around 30-35K, but without a hoe I think they are around the cost of a small grand L with loader.