Had both for quite awhile, both very different phones and very different experiences.
Beats isn't horrible - it's a gimmick unless you need a set of headphones, which I did. And yes, there's a lot of bass, but depending on the music I didn't find it to be over done too often - to each his own.
Rezound screen - more accurate, much higher resolution screen, hard to see outside, but absolutely beautiful overall.
Bionic screen - very subjective... some love, some hate, but you can't deny it's brightness and ease of use outside.
Battery life - I found these two to have the best battery life out of all the latest LTE phones - on par with each other, but the Bionic probably slightly edged it out on an average day for me.
Data connection - My experience was that the Rezound's radio and ability to hold a 4G signal (and switch instantaneously) was head and shoulders above the Bionic's, even after the 5.5.893 update for the Bionic.
Sense 3.5 vs. "Blur" are very different experiences. Apples to Oranges in my opinion, you'll have to decide.
Hackability... obviously the Bionic has a lot more going for it in this arena right now. But I know for a fact that devs are working hard on getting S-off/perm root on the Rezound - and once they get it, the phone will be a beast with that HD screen.