Speed, Camera, storage capacity (64g), better wireless antenna functionality... need I go on?
I had the 4, I have the 4S...
Speed is, for me, unnoticeably similar. Maybe a TINY bit snappier, but overall very similar. Camera and video haven't really changed a ton from the 4, either. Yeah, they are higher resolution, but resolution doesn't determine quality. Quality wise, they are very similar to me.
I have the 32, and only because of the video and pictures being higher resolution. Never would need 64.
The antenna functionality I guess is a plus, but I never had issues with the 4, so it's just peace of mind thing for me. The only thing that really is different is now I have the white iPhone instead of the black. The white is definitely sexier. But I coulda just gotten the white 4.