The thing with the Galaxy S devices is that the size is so odd (not in a bad way). When using them (for me the Vibrant) next to a 3.7 inch phone, the screen looks so much larger, but the phone doesn't feel too big. Kind of like a 3.7 with a case on it. When I held the X next to the Vibrant I had with me, the X's screen did look bigger, but the difference didn't appear as noticeable from 3.7-->4 inches. Yet the X feels a good bit more hefty. Not sure if this is just me...
Software wise they pretty much do the same things, with different interfaces. I like the clean look of TouchWiz, and yet I like the kind of rustic image of Shadowblur or whatever people call it. Both have their lag issues though. Although I do have to say TouchWiz is more customizable.
The still pictures on the X were better while I was testing, but the 720p on the Captivate was without a doubt the winner.
Also, the Captivate comes locked down by ATT, but you have the option to root to enable the sideloading of ups, elimination of TouchWiz lag, or flashing a custom ROM, as well as other functions. The Droid X, as you have probably heard, *supposedly* has an encrypted bootloader and other little tricks that keep you from flashing a ROM onto the phone. For some it's an issue, for others it is not.