The accounced changed (with photos) are in the news section www.mini2.com/news although I'm sure during evolution we could add a lot more to that. Is it between model years you're interested in, or certain MINIs made on certain dates? The reason I ask is that many changes and tweaks have been added at different times. The only MAJOR changes so far as model years go is the '05 against the '04, which are in the news section.
Although I drive the diesel, which didn't get the exterior changes, I have so far noticed that the 2005 seems to be better screwed together and quieter than the 2004 model(also duesel) I traded in.
Thing I miss most is the clock, keep looking for it on the roof, where IT SHOULD BE.!! Other changes like the larger rear-view mirror and extra lighting in the doors are less significant and wouldn't sway me one way or another.
If you can find a used example with a spec that you want then I personally would go for a used mini, but and it's a big but, having a car custom specced for you is a joy to collect from the dealer.