I have no idea why people prefer to kill ice giants over hill giants for prayer XP. I personally kill hill giants, but here is my theory: better drops, more combat xp (besides prayer), and less competition if you're using the edgeville dungeon. I personally don't really care about drops, i made 30k from their limp roots in 1 hour and got 1 ranged level (58-59) and 1 prayer level (42-43). And if they know what they're doing they can usually get the hill giants before anyone else can (i can). It's been a while since I've been down to the underground Hill Giants, but I do know that before Jagex implemented the minimum 1000 total world, the Hill Giants were ridiculously busy; what sucked more was when you had to compete for Hill Giants. While it is fun beating people to a Giant, it slows the process down when you begin to compete with other arrogant rangers. Most people use Ice Giants for ranged exp., as Animorph said, but yeah, I think Wildy Hill Giants would be the best for some quick bbones. And w113 Hill Giants if the time is right (?).