I have a B2710 and can only speak for it. I'm extremely fond of it and wouldn't trade it for any color of tractor. Everything about it is quality. The more I use it the more I appreciate the quality.
Before you buy a tractor test drive each one - on your land if you can get the dealer to bring you a demo. I doubt there is even $200 difference in these tractors based on what I have read on here and other boards. So I would pick the one that feels right, has the controls where you like them, has the right HST feel, etc.
The brake pedals on the Kubota are on the right side of the tractor - same as the HST pedal. I have heard some tractor owners who like to use the brakes for turning alot, which is more difficult when the pedals are on the same side as the HST pedal. This has never been a problem on my tractor as I have never felt the need to use the brakes for turning.
I'm not sure that the specs are apples to apples on weight, but the JD 4200 is a heavier tractor than the B2710, with about the same horsepower. That could mean it is underpowered compared to the Kubota, or it could come in handy helping you move stuff, as less
ballast would be needed. I prefer the lighter tractor, but I use my tractor a lot for mowing, and the heavier JD would tear up the grass more, while I could add ballast to the Kubota if needed. Kinda hard to remove basic weight on the JD to make it lighter.
Just some thoughts that might help.