I had the Thunderbolt and got the Rezound for $50 on upgrade on bf. If I sold the bundled headset, then it would be similar to getting the Rezound for free, and for a free upgrade, I think the Rezound is worth it if you like essentially an improved Thunderbolt. Plus, it's nice to have a spare phone around, it's cheaper than buying warranty.
Display - 384,000 vs 921,600 pixels. The very small text on my Rezound is sharp, same size text on my Thunderbolt is fuzzy. Overall though, a noticeable substantial but not a huge improvement.
Camera - I think the Rezound is slightly better plus it has 1080p video recording (Thunderbolt has 720p).
Speed - Rezound is slightly faster. Again, noticeable substantial but not a huge improvement. CPU intensive apps should run faster, but if you are just reading e-mail or browsing the web, then probably not a huge difference.
Data Connection - I have found that the Rezound has better data, especially because I am sometimes in an area of weak signal, and the Thunderbolt would sometimes drop out and take a long time to recover. Speed wise, my peak on the Thunderbolt was about 25mbps, on my Rezound, I was able to get 44mbps. With the latest radio, the Thunderbolt radio is good, but personally I feel the Rezound radio is slightly better.
Storage - The Rezound has 16gb internal/16gb sd card, which translates to about 2gb on phone, 10gb internal, and 16gb sd card. Thunderbolt has about 2gb on phone, and 32gb sd card. So you can add the internal storage with the sd card for a little more storage.
The Rezound also has other things like beats, mhl capability, which may or may not matter to some people.
I am not saying that the Thunderbolt is bad, but to me, stock for stock, the Rezound is slightly better in almost every area.
Of course, the Thunderbolt has better dev support if that is important to you.
The gn is a great phone, but there seems to be quite a number of people complaining about signal problems, just something to consider.