Go for the 15' if you want to do some text editing. The 13' is not serious enough to work, so to speak, more like a toy. 15' is 2x better if you work on two A4 size documents the same time on screen. 1280x800 just not enough for dual window editing. Even 1366x768 is not enough.
I can testify to that. Right now I have a Dell Studio XPS 1640 with a 16" 1366x768 screen and it is NOT enough. I can't wait for when I get my MBP 15" for the 1440x900 resolution alone.
Seriously though, there's a possibility of new MBPs on the 27th, I'd at least wait until then in case they do shake up the lineup a bit.
Edit: Wow, I did not realize how old this thread was. Someone please kill this post...