I have a Desire, an my roommate has a SGS.
after comparing on every aspects without root, i will still go for my desire.
why?
*desire has more solid case, while sgs feels more like a piece of 'plastic', sorry no offence here, but its true..
*desire sense ui wins, sgs stock ui is laggy (without lag fix, which required root)

and about the advantage of sgs, it already mentioned above.