(Comparison Disclaimer: I am not a "fanboy" of either HTC or Samsung. I'm coming from an OG Droid. I actually wanted to stick with Motorola, but I don't care for their latest offerings.)
I bought the Rezound 3 weeks ago expecting to return it when the Nexus came out. So, I played with a Nexus yesterday and left surprisingly underwhelmed.
The first thing I noticed was that the colors were way over-saturated on the Nexus. The Rezound definitely wins here.
I played the same youtube clip on the store's Nexus and my Rezound: Again, the harsh colors on the Nexus made me favor the Rezound. Also, the video looked more pixelated on the Nexus. Rezound won again.
Web browsing: It was a tossup. In some respects text on the Nexus looked better, but in some cases I preferred the Rezound. Again I was bothered by the harsh colors on the Nexus. I did get frustrated with the soft buttons on the Nexus. I couldn't figure out how to search when I was in the browser. I always had to go back to the home screen and use the Google Search widget. For a new version of the OS, I would expect the phone to be more intuitive, not less. I assume I'd eventually get used to the UI, so I call the results of the browser comparison a tie.
Size: Before I bought the Rezound, I was afraid I wouldn't like the size of this new breed of larger phones (coming from an OG Droid). I found the Rezound was just a tad larger than I'd like, but it was much more usable than I thought it would be. The GN is even larger and while I think I could live with it, I prefer the size of the Rezound. I also much prefer the feel of the Rezound. I actually like the extra heft to it. I even prefer the feel/shape of the Rezound with the extended battery, though I don't like the weight. If the Rezound had the shape of the phone w/ extended battery with the weight of the phone with the standard battery (did that make sense?), I think it would be perfect. So, the Rezound wins here again.
Phone reception: This is a huge factor for me (and it annoys me that most phone reviewers don't seem to care how the phone works...um...as a phone.

I've been disappointed with the reception of the Rezound compared to my OG Droid. So, I was thinking if the Nexus had better reception than the Rezound, that could sell it to me by itself. Unfortunately, there was very good cell reception inside the store, so I couldn't compare how they do in a poor reception area. However, I did run an app that reports signal strength and my Rezound always showed a higher signal strength. Granted, that may not translate into better reception, but it was something to note. I also noticed that the Nexus occasionally dropped from 4 bars to 3 while I never saw the store's Rezound go below 4. Without a good test of a poor reception area, I can't draw a conclusion on this comparison.
GPS: Ran GPS Status on both. The Nexus saw fewer total satellites, but locked on more than my Rezound. Also, while my Rezound showed it was locked on a number of satellites, it wasn't reporting a location. This made me think the Nexus was superior. However, Netmonitor always reported higher GPS accuracy on the Rezound. Perhaps I didn't wait long enough for GPS Status to report a location. Then again, I've seen some odd GPS behavior on my Rezound. Further testing needed.
Audio: Beats Audio is a gimmick. I'm an audiophile-wannabe and Beats-Audio-enhanced music sounds terrible to me compared to non-enhanced music. Beats Audio is not a selling point for this phone (except that you can sell the earbuds). Regardless, I am concerned over the "static" heard when using headphones on the Rezound. I did a short test and I heard it occasionally on my phone. I didn't try it on the Nexus, but I haven't read any complaints. So, I'll say the Nexus wins here.
UI: I don't care for Sense. It has some nice features, but it's bloated and I find its customizations take away more than they add. I would much prefer stock Android. The Nexus with stock ICS was nice, though not as intuitive as I expected. Still, Nexus clearly wins here.
The big advantage the Nexus has is stock Android (and ICS to boot) and is easy to root and get custom ROMs. These features alone are very tempting to me, but I can live with Sense if I have to. I'm really hoping the Rezound will get rooted and get custom ROMs, but again, I think I live with Sense.
So, again, unless I can see evidence the Nexus clearly does better in poor reception areas, I'm going to keep the Rezound.