Griff... It's kind of a tough call. The UPZs are 28s, and the Stratos Pros are 29s. The one thing I didn't like about the UPZ is that they're pretty tight in the foot (but I think i'm changing my mind on that) The Stratos are really comfortable boots with a thick tongue that helps prevent possible shin bang.
The only problem with the Stratos is that they're a bit too roomy, probably cause they['re probably one size too big for me. 28s in those would probably be my choice of boot. They also come with two tongues(the UPZs only came with one... stiff), a stiff and a softer one. I'm riding with the softer one right now, but I'm probably going to switch to the stiffer ones just to reduce my movement.
So in otherwords, if i had equally sized boots I could give you a more precise comparison..
One other person on this site said he had done a direct comparison between the boots (equal sizes) and found that the UPZs were tighter in the foot where the Heads were roomier. I agree with that.
Which I like better... right now the Heads even though they're a bit big. But I just started this season and haven't had very good runs in the UPZs. I may have a different preference after this weekend. :D