Do you live in a strong 4g area? Why do you want to switch from your Bionic(just curious)? What exactly are you looking for in a phone?
I can only vouch for the Nexus of course. I live in a strong 4g area and have never dropped a call or internet, well internet once or twice, but it came right back on, probably lost it switching towers. The battery on the stock ROM/kernel is mediocre at best on the Nexus and the same can be said about the Rezound. Both have big beautiful screens so it's pretty much a tie there, though I did notice that the Rezound has certain angles where it's not as visible, could be an advantage or disadvantage.
The only knock I can really think of against the Nexus are its weak radios, which I noticed when I was down in socal. Ended being stuck on 3g most of my time there but it may be due to the area, as alot of people don't get too good a signal down there. Other than the radios, I really can't think of anything to put against the Nexus, other than battery life, which can be fixed with ROMs.
If you are into rooting and ROMing, this isn't even a debate, the Nexus wins, hands down. Need more convincing? Jelly Bean.
Both phones are very good devices. Out of the box, stock for stock, I'd have to give it to the Rezound. However, if you did get the Nexus for rooting and ROMing, really no competition, especially with JB ROMs based off AOSP now.