I think it totally depends on your brother personal preference and what kind of a user he is. Every OS/phone has its strong and weak points.

I never really have real long term experience with Android but from what I played with my friend's Galaxy and NITDroid, it's very good and looks very promising.

IMHO, iPhone 4 would be the best candidate if your brother is the type that doesn't demand much from the phone and want a lot of apps/games which are just there to download (don't have to mess around much) considering the over-hype around iPhone 4 in Aus right now (I'm living in Aus at the moment). I think you can get 16GB iPhone4 unlocked from apple store for AUS$ 859 and 32GB for about 1K.

As a non geeky n900 user, I'm very happy with n900 . i think OC following the guide in here is much easier than trying to make iTunes work the way it should ^0^.

The above opinion is purely my personal with non-technical reference. Please don't bash me for it.