The X10 is running 1.6. It CAN be updated; whenever Sony gets around to it. 1.6 has been out forever, 2.1 is becoming old and 2.2 is almost a placeholder for Gingerbread (3.0) supposed to hit sometime this year. The hardware is nice, but the lack of multi-touch is a massive pain and the Timescape applications bog the phone down like none other. When it was announced, it looked like the best phone ever. But by the time it hit the market, it was old, and by the time it hit the US it was REALLY old.
The Captivate has the best GPU in a phone right now. It has a very strong modding community on XDA, and the 2.2 update is scheduled for October. The SAMOLED screen pops and makes everything else look washed out by comparison, and the GorillaGlass combined with the thin and light body make it easily pocketable. The camera is not as good, and is missing a flash, so that can be a dealbreaker if you're into taking photos with your phone that you plan to show off (rather than reference, which is what mine is to me). TouchWiz is as bad as the Sony UI, but can be avoided with an aftermarket launcher (and with root and ripping out the AT&T bloatware it SCREAMS). The physical buttons on the Xperia are a plus for sure; the capacitive ones on the Captivate don't seem to be as responsive and you're often wondering if you've hit them or not.
The Xperia just doesn't have much going for it as late to the game as it is. It won't be worth anything till it's updated (better to skip right to 2.2 honestly), and even then you're stuck without multitouch which is a dealbreaker for me. If you like the camera, and don't mind the sluggish UI, then it could be for you. It isn't for me.
