If I didn't have 4WD around here the tractor would be at the bottom of one of our steep slippery hills, and that would be where it would stay.
Think about this, When you pick up a load with the FEL (Front End Loader), guess what happens? Your rear tires get light, and if you are pointed down hill, they get VERY light. Just when you need them the most. Now you have reduced traction. Sure you can compensate some by loading the 3PT hitch with something heavy and that is highly recommended to do even on a 4WD tractor, but it is still not the same. Really big farm tractors with really large wheels and tires, can do much better with a loader because they weigh so much to start with and because of the size of the tires, but an MX5100 does not qualify as a "really big farm tractor". It NEEDS 4WD to really be effective with a loader. And the more hilly or wet your terrain is the more important it becomes. Do what ever your heart and pocketbook tells you, but I think you would be making a BIG mistake by not getting the 4WD on the MX, or any tractor that size with a loader.