I think anything above maybe 30hp will do the job, but the bigger you go, the faster most of the work will get done...it comes down to how important time is to you, and what your budget will handle.
As far as brands go, both companies make great machines. The best thing you can do is get yourself some seat time on several of them, and see what you think. Things that don't bother me might really annoy you, and vice-versa. It'll come down to where things are placed, the seat, etc, etc.
When Kioti came out with the NX line I spent some time in one and was pretty impressed at what you get for your money. The fit and finish was excellent (this was a cab model) and it appeared to be very solidly built. If I was buying something in the 40-60hp range with HST, the NX would be my first choice. I wouldn't turn down a free Kubota, but they tend to run about $10K more for similar performance/features, and there are a lot of things I could do with that $10K. I don't have any doubts as to Kioti quality, and would fully expect thousands of hours of trouble-free service out of one. Any machine can break, but between these two companies I don't see either as being any more likely to have problems.
