Having had various Android phones as well as an iPhone, I more than happy with my HTC Desire. Was going to upgrade to the Desire S as it's even better, but now I'm waiting for the HTC Sensation which really (down to size) is a replacement for the HTC Desire HD.
So from my experience... smaller phone Desire S
larger phone Sensation out later this summer on Vodafone then the other networks a month or so later.
Go play with them in a shop tho before you go for it!
Also, once you have one. If you don't like the user interface, you can change it, unlike those iPhones