I had the Rezound on its launch day up until the Gnex on its launch day. I've been using the Gnex ever since then.
Quite honestly battery life seemed to be nearly the same, if I had to give an edge to one, it would be the Gnex.
The screens/display for each was nearly equivalent. Again, very hard to pick a winner, but if I had to I would say Gnex due to its large size when the soft keys disappear for certain apps, etc.
What made me choose the Gnex over the Rezound:
1. Fast updates -- you get your phone updated very quickly (even if you dont end up rooting/ROM'ing)
2. No Bloat -- although it didnt really bother me while I had the Rezound, I could foresee those apps slowing me down over the long haul.
3. Unlocking/Rooting/Rom'ing possibililties -- Dev community has got to be the largest around. I had been toying with the idea of rooting my next phone to be able to put different ROMs on it. Also, I dont have the fortune to be able to get a new phone every 6 months (give or take). So having my phone rooted with the possibility of putting a new ROM on it whenever I want gives me slightly more time without the phone getting "old" because you are constantly putting whatever is newer and better onto it. I basically get the fun of a new phone when you move to a new ROM or update to the current version of the one you are on. You can now easily unlock the Rezound at HTCdev though, and most likely root it (dont quote me on that, I havent stayed current in that department).
4. Ice Cream Sandwich -- If you arent going to be rooting, this has got to be the biggest reason I would choose the Gnex. It has the newest iteration of Android all ready to go, no waiting required. Who knows when the Rezound will get an official OTA update for ICS??
What I will admit the Rezound has over the Gnex:
1. Sound -- the Gnex speaker is quite pathetic. It irks me that I had to install Volume+ just to get my speaker loud enough so that I wouldnt miss notifications or phonecalls anymore.
2. Weight -- I liked the "heft" of the Rezound, but just because the Gnex is lighter, it doesnt necessarily feel "cheap" to me like some may suggest.
That's my 2 cents. Obviously its an opinion, but ultimately you need to choose what will work for you. Go play with them a couple of times on a couple of different days. They are both very good phones. Good luck with your decision!