Yep - Samsung Galaxy S. I've played with the Desire and Samsung Galaxy S today in a store for about 10 minutes, side by side. The SGS is soooooooo nice to use.
I gave up on the Xperia X10 about 3 months ago. It sounded great when it was announced, but then took 6 months too long to come out. By then, most other Android phones had topped it in all aspects other than screen size. The Xperia X10 is slower because of its custom UI being slow, I personally didn't like the Mediascape thing, and it's stuck on Android 1.6, which by the way means the screen can only use 65,000 colours of the 16M available. It also has less memory than the Desire and SGS. To be honest, I think the Xperia X10 was outdated about 6 months before it was released because it just took so long to come out.
Sure the camera is probably good though. While the Desire is also 8MP the HTC cameras are pretty crap compared to the Sony ones.
I'd seriously look at the Samsung Galaxy S, it has the best screen on any mobile phone anywhere in the world, end of story, and it's the same size as the X10.
Sense UI is nicer than Mediascape, and nicer than Android 1.6 standard UI.
TouchWiz on the Samsung is nicer than Sense in my opinion.