its u r planning b/w these two ,then have a look at samsung galaxy s i900.it is one of the best android smartphone.it is far better than x10 and desire both.
and out of these two htc desire is a much better option as compared to xperia x10.
htc desire has an amoled display with multi touch feature whereas x10 has ordinary lcd display with no multi touch feature.
sony is good wid 1gig processor don kno abt desire, i think htc has 1 gig processor only in nexus 1.. n u can easily update ur firmware to v2 or above..
guys,,,, iphone 4.... thts it end of show... there is no better touch, processor, quick, applications, features, media, hd video recording, anti scratch display, or watever u looking for....