I was trying to decide between a 13" MBP, 13" MBA, and 15" MBP. In the end I went with the base 13" MBP. If this was my only machine, I would have went with the top-end 15", but my main machine is a 27" iMac, so I decided to just get something cheap and portable. Cheap in the Apple sense, that is 
13" is a small screen. I have found that 15" is pretty easy to live with, but with 13" I have to jockey windows around a bit. So I use full-screen mode and create additional workspaces in Mission Control. I'll probably set my dock to auto-hide too, that's something I've been meaning to try out.
Anyway, sorry for the tangent. Your questions about pros and cons can be easily answered by comparing specs. You'll just have to decide which of those pros and cons are most important to you, and then it will be an easy decision.
13" is a small screen. I have found that 15" is pretty easy to live with, but with 13" I have to jockey windows around a bit. So I use full-screen mode and create additional workspaces in Mission Control. I'll probably set my dock to auto-hide too, that's something I've been meaning to try out.
Anyway, sorry for the tangent. Your questions about pros and cons can be easily answered by comparing specs. You'll just have to decide which of those pros and cons are most important to you, and then it will be an easy decision.