I will try to be concise, but I may run long. This is what I have found between the two phones. As a disclaimer, I did pick the Rezound.

I have the feeling that phone hardware has been able to achieve a general plateau that will allow for it to run for longer periods of time and fewer problems than what we saw for the G1 & the OG Droid. With that in mind, the lifespan of these two devices really are not going to matter as much since they have the exact same capabilities in RAM, ROM, and processor (dithering about the processor and graphics chips at this level is dropping to the level of minutia imho).

Now, aside from lifespan, you are looking at support. The GNex will probably have more support longer being a Nexus line of phone. However, if Cyanogen picks up the Rezound as a device, my opinion is that support is a null issue. Yes, GNex will get updates sooner, and have more ROMs available, but the Rezound will still have ROMs and decent support.

The GNex appears to be the sandbox phone.
However, some things I have noticed is also the forums. Here on Phandroid, the general tone of the forums are completely different. The GNex forums are a bit more complainy, where as the Rezound forums generally are happy and helpful. I believe this is due partially to people's expectations of the GNex. Small problems (and what seem to be some moderate to large problems) easily gain momentum and can feel very painful.

The removable memory is more of a personal taste. I like having it, I think more of the fact it is an option than any specific actual use
In regards to the camera, I really believe some of the awesomeness of the GNex's camera is OS based. With that said, and HTC said the Rezound will get ICS...I believe the Rezound's superior camera (it has an f/2.4, almost exactly like the iPhone 4S - I'm not referring to MPs) will benefit more from the OS than the hardware in the GNex.

I don't really like HTC Sense UI, but I can live with it and work with it. It is not the devil, just something different
For batteries, my Rezound can give me a day's use easily. The fastest way to do this is know if you are in a 4G area and turn it on only if your home/work/etc locations are good, otherwise set it to 3G. Secondly, I use Taskkiller. Not to free up memory, but to make sure those stupid bloat-ware things are off and stay off If I don't browse the web and only use for phone calls & checking email - I can get it to last 24 hours! That's not a "smartphone" though

Oh, and the GNex is a Nexus device, it doesn't have Samsung's UI on it Straight vanilla Android like the OG Droid.


In regards to outstanding issues:
Rezound you can now unlock the bootloader and get root, but you can't do traditional ROMs like the OG Droid because of the "S-On" (security settings, like BIOS lock). Also, some Rezounds have a fucia tint instead of true red for bottom buttons (unless something else is wrong, I wouldn't return it - and I think I have one like that ) and the back can get warm, but it shouldn't get burning hot, when running high processing stuff like streaming video.

The GNex appears to have an issue with signal strength. There was also a touchscreen issue with games, but that may have cleared up. I'm not sure of the current status or if there are others since I stopped following it about two weeks ago, but it would be good to check their forums and over at XDA for better info.

Oh, and one last thing, the other item that really made me choose the Rezound was the buttons on the GNex for power and volume. Just felt backwards...but I do use my left hand for the phone calls, but make sure to hold one and try to use it a little in the store before you decide.