The average American uses about 2gb a month (source: it's on the internet somewhere) but it depends hugely on what you use. Games, movies, and streaming music suck down way more data than anything else.
Personally, I like android because where iPhones are consumer electronic devices, android phones are hobby kits. And I don't use data plans at all anymore. I find paying piecemeal for data insulting somehow and wifi off my home internet works better. (And is useless if one is on the move, but I don't use services that require constant updates, so whatev.)