Depends on what you are going to do. If you are just writing papers, any model will do, and I'd suggest you even save and pick up a refurb / '09 model for that work.
If you are going to get into graphic design, multimedia editing, CAD, or business/game development - you'll almost certainly want an external monitor regardless.
So I'd suggest either a 13 or 15" - with a 24" External monitor for when you are doing your "serious" work. 🙂 I do development on both my 2010 13" and my 2008 15" - but when I have a lot of editing or spend significant time in Xcode, I'm always using my 22" or 24" in combination with my MBP screen.