RE: Which Summit stand for bowhunting?
Wow, golly! Varmint, you've been given some excellent input from these great folks once again!
I, too, own the Viper XLS and the Cobra XLS top, so I can choose whichever one I want to use that given day. I'll tell you I only used the Viper top once or twice last year, and every other time I grabbed the Cobra top.
You definitely want the backrest; I've used fixed-position stands my whole life up until a couple years ago (where, like you said, I just used the tree for a backrest), and that padding back there makes a WORLD of difference in comfort! As noted, it also QUIETS the noise down, as you won't get your shirt or anything snagged on the bark.
Even though I prefer the Cobra XLS for bowhunting, I'll try to add one advantage which hasn't been mentioned previously. If you're using the Viper, you can lay your bow across the bar for easy access. Just kinda nice to have it that close and not have to turn around and reach for it off of a limb or screw-in hanger or something.
Bottom line is, you really can't go wrong with either. Even if you bought one and was still wondering about the other, you can do like BrianDamage and I did and get the other top; then you've got the ultimate in selection at your availability!
Discipline -- not desire -- determines destiny.