I just returned the TBolt...my main reasons for doing so:
1) Like seattle above, in my very unscientific comparisons with the Thunderbolt on 4G and the X on 3G, internet pages loaded about the same. However, application installations from memory card were faster on the Thunderbolt...this is where I think the 768 RAM was of benefit.
2) The Thunderbolt seems to not get as good a of a signal as the Droid X...I had both phones in a restaurant last week and the Thunderbolt was hot to the touch (literally) from signal searching, while the Droid X was cool and holding a strong (albeit low) 3G signal. The signal on the X is the best I've seen on a smart phone to date.
3) The X has far bettery battery life and I don't have to turn everything off. I have GPS, bluetooth and wifi on all the time, use it "at will" (in lieu of trying to be "conservative") and this phone with the 1930 battery in it literally lasts as long as I do...which is all I have ever wanted from my smart phone. Get up when I get up, go to bed when I go to bed.
I wanted to like the Thunderbolt but I actually also prefer the form factor of the X...and believe it or not I prefer the SCREEN on the X. I'd rather give up some of this fancy screen technology for a screen that just works in bright light as well as low light conditions, and doesn't chomp the battery at 80%+ use. The Droid X, to me, found that perfect balance of a nice screen yet it doesn't use nearly as much battery as the "super" screens, etc. I find it perfectly fine for everything I do from watching movies, to playing online games, to working on word and excel documents.
I am SO impressed with this Droid X, I think my next phone will be the Bionic. If they can ace the battery life I am on board. And if they don't, I have no doubt I'll continue on with my X and be pleased just the same.
Good luck with your decision.