Did you see Amvcane's post earlier today? There is simply no reason that HST should have any more difficulty going up hills than a gear transmission. Yes, you lose perhaps a couple of HP to the tranny compared to gear but there is nothing about hills and HST that is different than gear. What does happen a lot is that folks treat the HST "go" pedal the same as an accelerator pedal in a car...doesn't work that way. Backing off on the "go" pedal actually the best way to climb or descend a hill. Back off just a bit as you start to climb, yes the tractor will slow but will keep going. Mashing the pedal to the floor is kind of like upshifting to 6th gear in your car when you hit a big hill.
Another reason to prefer HST if you are in hilly country is that HST is MUCH safer for descending hills. You control your speed easily with the go pedal. The first reaction of most folks when going too fast in a gear vehicle is to put in the clutch...that can be a disaster with a geared tranny on a hill. Likewise, I would only change range on an HST on a hill after coming to a complete stop with foot brake on. Moving from one range to another takes you through neutral.