Here's my take on it, as someone who has been researching both phones for well over a month now.
Screen: PenTile vs. LCD, and it probably boils down to personal preference. I think most owners of the Rezound favor their display, while most people waiting on the Nexus say that display looks better. Only way to decide is to see for yourself. With the buttons on the screen, the Nexus and Rezound have pretty much the same size screen. The Nexus can only claim the 4.65" when streaming video.
OS: Vanilla 4.0 (Ice Cream Sandwich) vs. Sense 3.5 (Gingerbread), and it is another personal choice. People who have been using Android since the beginning are raving about the changes in ICS, but newer users (or non-power users) seem quite content with the HTC Sense overlay on Gingerbread. Again, the key is playing with both and seeing what you like. As someone who has never owned an Android device, I can see the benefits in both. I like a clean OS (Nexus) as it will not lag the phone's hardware as bad as a "bloated" Sense-skinned OS (Rezound) will. That said, both phones have powerful processors, and I'm reading that lag on the Rezound is minimal (probably unnoticeable to an Android noob like myself).
Hardware: OMAP processor vs. Qualcomm processor. From what I've read, the Qualcomm processor is "the worst dual-core processor" available. Take that for what it's worth, but I'm not sold on there being a "bad" dual-core processor. I compare this to the Intel vs. Athlon debate. I believe Intel to be the better chip, but I run Athlon for its cheap power and the fact that I am not a power user, and I've had great results. While the OMAP processor in the Nexus is not top-of-the-line, it is reported that ICS has been optimized to run on this hardware. So, even though the Rezound will get ICS in a future update, it probably won't run as smoothly as the Nexus does today.
Other:
Too many variables here. Removable SD card is kind of a big deal to me, so points for Rezound. Updates coming straight from Google (promptly) is points for Nexus. I'm too new to Android, so rooting and ROM'ing is out for me right now, so I don't really care that the Nexus will have the stronger development community support (push for me).
Anyway, long winded post, but hopefully it helps. If you do a web search for "HTC Rezound Reviews," most reviewers are already comparing the phone to the Nexus. The only reason I'm still waiting on the Nexus is because I can't find a deal on the Rezound. Had I jumped when the Black Friday deals were going, I'd have a Rezound today. Discount on the Rezound makes this a very easy decision for me.