I've had the Desire, i now have the iPhone 4, i personally prefer the iPhone, the Desire still felt slightly choppy to me, not as smooth when scrolling or when on the internet. Also rooting doesn't really allow you to do that much to be honest, whereas on the iPhone jailbreaking unlocks a whole host of things.
One of the main reasons for me to sell my Desire was because of the lack of useful apps, there maybe loads of free ones but most of them, to me, were quite useless, and apples app store has many more which gives you a wider selection, sure they have useless ones as well but to me there is more useful ones also.
Also regarding the apps, most developers and big companies bring out apps for the iPhone first, or in some cases don't develop them for android or other platforms at all, such as if your with orange, the Your Orange app, which allows you to monitor your minutes, texts and internet data is only available for the iPhone, which was very annoying.
If you need any more information just ask.