Depending on what all you need to do, I find that it is hard to have too big a tractor when you are looking in this range. I would lean towards the 4120 myself - it is not too big and is a lot more tractor. With that said, it might be too much tractor. I bought a 4720 to maintain our 20 acres and it has worked out great. You might want a i-match for that mx-6, it is pretty heavy duty.
Also, if you don't need the heavier duty MX mower, you might consider the LX to save on costs. I went with the MX because I have a lot fo nasty mesquite trees that needed chopping up!
Let us know what you end up doing.
I agree on the rental thing. I hate having to wait in line, delivery, etc- I can jump on the tractor and go when I want to do it.
D.
D.