Android -> Linux based, 150,000 apps or so on their store. Good because it spans multiple phone manufacturers and provides a simple but feature-rich OS in a smaller price bracket.
iOS -> Based on OS X, a more premium OS, and has 300,000+ apps. Better (IMHO) because of it's insane simplicity (proven than a 3 year old can use it) and also, like android, its feature rich-ness. But it is only available on the iPhone which is very much a 'premium' piece of kit.
Hardware wise, using the Galaxy S as a rival to the iPhone -> CPUs are near enough the same, the Galaxy is expandable because it uses removable memory cards rather than embedded flash memory. The RETINA Display on my iPhone isnt quite as 'big' feeling as the AMOLED on my girlfriend's Glalaxy S but my resolution is definately better.
Its too general to just say 'android vs. iPhone' because thee are 80 quid android phones and there are 600 quid android phones. A Sony experia is no match for the iPhone, but the Desire HD and Galaxy S/Ace definately are right up there with it. the Nexus S also.