I have a Droid X and a Samsung Captivate. I also have an iPhone 4. I voted "other" because I like the Captivate the best of all the Android phones I have. It's also very sleek, thin and has no physical buttons, a real plus as there is nothing to collect dust or pocket lint, or fail.
One thing the OP did not say is if a physical keyboard is of importance to him/her. As a long time BlackBerry user, it's the first thing I missed. Then with the advent of Swype... and the Samsung Captivate, I find that I can create emails faster than I could after several years with a BlackBerry. I never thought I would say that as I was quite dependent on a physical keyboard.
Not only is Swype an amazing and fast input method it's fun and very easy to use. Typo's I used to make on the BlackBerry are a thing of the past with Swype.
However this is the exciting aspect of new technologies, as there is usually something around the bend that changes everything. Swype is that killer app for me. If not for that, I would most probably still be using a BB for my messaging. Especially given the fact that I handle about 40 emails per day from the smartphone I carry.
As much as I like my Droid X with the 4.3" display, and as one that places keyboard and display features above all else, the one overriding factor that makes the Captivate more appealing to me is the Super AMOLED display. It's so much clearer, brighter and more vivid that the TFT of the Droid X, the .3" difference in size is simply not worth it. At and even 4.0" the display of the Captivate seems to be the sweet spot.
Funny what we get used to, suddenly the 3.5" display of the iPhone 4 seems positively tiny.
Cheers...