It's really going to come down to preference. Both of these phones look really great on paper and compare well to each other, at least from the rumors.
Do you want a vanilla Ice Cream Sandwich Android phone that gets cutting edge updates directly from Google? Go with the Nexus.
Or do you want a gingerbread packing HTC Sense phone and wonder when the hell you're going to receive your updates? Go with the Vigor (Rezound).
Both of these phones will be beasts. Both will be packing cutting edge tech and LTE. Both are made by manufacturers who have had questionable build quality in the past. One of these phones will be packing the newest version of Android, the other will not. Has Verizon and their manufacturers figured out the LTE battery life debacle? The Bionic battery life is decent, can Samsung and HTC follow suit? I think there's a lot of questions to be answered about these two phones as well as the Droid Razer, but we won't honestly know anything until we have the devices in our hands. If you're unsure which way to go, wait til each are out 1-2 weeks and then read up on the reviews and make your decision then.
Me personally, will be going the way of the Nexus. I've been dreaming about Verizon getting a Nexus device since I first got my OG Droid. I never imagined it would be the last vanilla android phone on Verizon. Hopefully that will change with the Nexus. I want a cutting edge phone AND the newest OS, and the only way to have both is with the Nexus. Something about buying a new phone with capacitive buttons (i.e. Bionic, Razer, Rezound, most new phones coming out soon) seems like such a waste when we all know they're being phased out by Google in ICS. Just can't bring myself to buy one so that's also why I'll go with the Nexus.