IMO, except for camera, which is pretty good in Galaxy S and possibly looks, there is no other reason why you should go for x10.
X10 afaik has locked bootloader which lends itself harder to root and apply custom ROM's. Plus, no multi touch and 65 k colour limitation even after eclair update. Plus, very delayed froyo update if at all it comes.
Many Galaxy S users could run froyo, much before Samsung released it officially, thanks to the awesome developer community support Galaxy S enjoys. And I have not yet talked about Galaxy S's strong points !!!
Mainly awesome screen, best processor and gpu you can find in a mobile, bluetooth 3,wifi n,4 inch screen etc...
IMO, HTC Desire is a better competitor.
hopefully, the x10 successor (x12) won't face a majority of the issues, and run ginger bread