The 2.1 os isn't that much of an advantage. Sony will be releasing 2.1 or 2.2 for the X10 soon enough.
X10 is the bst phone because it has 8.1 mp camera and has the biggest screen, also looks nicer too, only drawback is software which will be fixed soon. Other than that it's the best out there imo, thats why I picked it over HTC.
I've had a good go with both. The HTC has the better feel and build quality because well, it's HTC. The X10 is let down by the lack of the trackball, 3 home screens and slightly unattractive default theme. My brother is considering selling his to get a Desire but is holding off due to the imminent ability to root it and install a community rom. After coming from an iPhone he loves Android but hates SE's implementation of it.