I had the same question the other week but I opted for the iPhone 4s. I don't really have a problem with the 4s's size and all the rumors I heard for the 5 were that it would be longer and thinner. The only issue I really have now is that the 5 is supposed to have a metal back while the 4s still has the glass. But I just invested in a nice case! As of right now I love my decision. I'm just kinda scared the 5 is gonna have some other feature I would really want
I honestly think that it will be mainly cosmetic... That and well iOS6 of course, but you will get that on your 4s and will be getting the new maps update as well unlike iPhone 4 users.