Ive had both, the Desire S is the newest and has an upgraded android platform,
the HD has a better camera and a larger screen (but the phone is huge because of this)
the S does everything (inc HD format Video) that the HD does and is a better size IMO for a phone.
i love the S and its cheaper.. ( brand new, poss not 2nd hand as its newer)
also, with android you can get most of the apps that are on the IPhone and others that are not..
the IPhone vs the World is a debate is one i don't want to get into, its just my opinion
hope it helps, (makes sence)
Will