I've used both and liked both. I am very happy with my Desire but as I have said on previous threads, maybe that is because I went into it with lower expectations.
Really you're looking at a prettier and smoother UI and more well-rounded App support on iOS compared to a more customizable, flexible and 'open' Android OS. Each has their own advantages and disadvantages but, truth be told, for about 80+% of the functions of a smartphone both OSes are pretty similar, really.
Updates are nice (and it's something Android and Android phone manufacturers can hopefully improve upon) but sometimes they can be problematic on old phones. Swings and roundabouts, really.
Apple have a more impressive legit gaming library, but if you were wanting to be a tad naughty, Android users have access to emulators (something I don't think Apple will *ever* support), so have a whole catalogue of Playstation, SNES, Megadrive etc games at their disposal should they wish to use that function. Again, Apple have lots to keep people entertained with though through their pre-approved stuff.
Really I find it difficult to make the decision between IOS and Android. As a pretty, sparkly, 'wow' factor piece of software, iOS is an incredibly decent operating system. In principle I prefer Android's approach even if it's more rough around the edges, because I think openness and freedom is more desireable a feature on a smartphone than a locked-down piece of kit. Of course, in day-to-day practice, for 99% of what I and others do, I'm not sure it's a big deal really, but that's how I make my cautious decision of Android over iOS.