I think it depends on how hilly and how abrupt the hills are. My dad's 757 JD, which I bought for him, does as well or better than a standard garden tractor on most hills--where he lives. Where I live, where it is not uncommon to encounter a 30 plus degree hill, one has to be careful. If a ZTR slips, it is curtains, because they are so difficult to steer through a controlled slide. My advice, if it looks too steep for a garden tractor, it is probably too steep for a ZTR. Moderate hills are fine and most commercial (not homeowner--commercial) grade ZTR's will handle them well. I would say 90% plus of us do not have anything that a ZTR cannot handle gradewise, but one must be as careful on hills as with a conventional tractor. I mow diagonally mostly on hills, and as for your question about the TANK--it is a good machine--but it is tough to beat a JD. My local dealer sells Grasshopper, CC, Exmark, Scag, Great Dane and JD, and for ZTR's, he says, "I like the green ones the best." Not to say the others are not excellent, but he sells a lot of ZTR professional mowing equipment and his reasoning seemed sound to me.
John M