We've got 50ac as well. About 15-20 is flat field, bordered by a creek and treelines. The remainder is spread over two decent sized hills and a sandy, wooded lowland on the other side of the creek. With all the trees & slopes, I wanted hydrostat to increase the maneuverability. At the time, the biggest HP hydrostat CUT was the Kubota L5030, at 50HP, and that's what I ultimately ended up with.
IMHO, it's the perfect size. I can mow with either a 90in finish cut mower, or a 7ft rotary cutter. I've run a disc up to 10ft. And I can still move the round bales that my neighbor bales for us. But that's the real rub -- I can't really run anything but the very smallest of hay equipment. But like I said, since my neighbor has all the gear, I figured there was no sense in us investing all the $$ in a larger tractor and more equipment.
As such, 50HP is just perfect for our 50ac. YMMV.