( Which vehicle will be best for going up and down some steep hills and blasting through high grass along side overgrown pastures? )
For going up and down steep hills, neither the HPX nor the RTV nor the Mule will work. I've driven them and have owned several Gators in the past as well as an RTV now. None of them will go up really steep hills. You also mention blasting through high grass etc. You won't do any blasting around in either the HPX or the Mule or the RTV. They are all decidedly slow.
If that is what your intended use is, you really ought to drive a Yamaha Rhino. It will climb ten times the hill an RTV or HPX or the Mule will. And it certainly will blast around! It also has the ability to haul small to medium sized loads. Since you didn't mention hauling heavy loads with the ability to dump them, the Rhino seems much more of a logical choice for your needs. The Rhino will also tow as much, if not more in actual experience, as either the RTV or the HPX or the Mule. The only "shortcoming" of the Rhino is in the size of it's bed and it's weight capacity in the bed as compared to the RTV or the HPX.
Since I needed a real work horse and also wanted something to blast around in, no one unit suited me. That is why I ended up buying two different units. For me, the HPX looks better, has a great local dealer, and has JD's great reputation. However, the HPX is priced higher than the RTV when priced out with similar features. The Kubota RTV also has a great local dealer, a great reputation, true hydro dump that cycles much faster, a heavier true hydro transmission, power steering, and a better ride. So, for my particular work needs, the RTV just was personally a better fit. As bczoom mentioned, the difference between the RTV and the HPX would be more of a personal decision based upon particular needs rather than a decision of which is a "better" unit. For me, both the HPX and the RTV are better at heavy work, but neither can hold a candle to the Yamaha Rhino at climbing steep hills or blasting around.