Those two tractors are close enough to be fraternal twins if it weren't for the paint! The green one may be a bit larger in some dimensions, but the orange one has a slight edge in some performance measurements, such as lifting capacity on the 3 point and on the loader. However I always say going just by those spec sheets is not a good way to choose a tractor for two reasons:
a) It's way more important that any tool feels and works right for its owner, even one as big as a tractor. That's a highly personal thing and what makes something preferable to one of us may be a complete turn-off for someone else. What difference does it make if one tractor lifts 100 lbs more than the other if you're struggling with controls that don't work smoothly and comfortably for you? Check out the operator station and controls, how each one runs and works; eventually your own judgement will guide you in making a choice.
b) Also, just like you don't drive your car at top speed in the real world... it wouldn't last very long if you did... you shouldn't expect to run your tractor full tilt boogie all the time either. Size your equipment so you can use it most of the time at, say 80% or less of its full capability. That goes for pto power, lifting power at either end, pulling power, etc. Your equipment will be safer and last longer and you'll have something in your back pocket for those unexpected situations. If your work ends up demanding 100% maximum performance from your tractor 100% of the time, well, you probably need a bigger tractor.
The one significant operational difference between the two tractors is the 2630 has a 3 range transmission; the 2520 has only 2 ranges. Most owners indicate that middle range is pretty handy since it allows you to stay in one range for most routine work.
The Kubota does have a slightly heavier bucket than the JD; I've not seen a flimsy bucket on any recent Kubota models.
Either make is world-class in quality and holds its value. Couldn't lose with either one, and you should also consider some the other brands also; there are many good tractors to choose from.