I have a smaller tractor than the one you're looking at, but my hills are pretty steep (up to 30% grade - where I
don't mow "side to side"). The most important thing on the hill is low center of gravity (COG). So you fill your tires, add weight (as low as possible), etc. You also want to set the wheels at the widest the tractor allows, so you might check the two models to see which has a wider track.
4WD is very helpful - you'll get some slippage (even with ad tires, which I'd also recommend) when going up, turning, and 4WD really helps. More HP is probably not important as far as the hills are concerned (in fact, I'd guess this is the one case where more might not be better).
So bigger, wider tires - yes. Filled - I'd say yes, but there are different schools of thought there. My next set of tires will be foam filled, for sure. And they will also definately be r-4s - every now and then I take my tractor mower out there and the turf tires will slip even when it is "dewey" (wet).
And get a tilt meter! I actually use mine more to assure myself that the tractor
isn't really tipping over, 'cause it sure feels like it wants to when the angle gets to 15% - 20%.
As far as tractor size goes, I have a
B7300 and wish it were a bit bigger. But not much - it would be too hard to get in and around trees, and I also understand that the smaller tractors are less susceptable to tipping (could be wrong there).
Good luck...Chris