The question is exactly what everybody else is saying, what are you planning to use it for, and is this going to be your main computer?
For me, I was also torn, but this is my main rig, which I am very happy with my purchase. I need ports such as Firewire 800, and Thunderbolt. I went for the 13" MacBook Pro, and the size is perfect to travel with, and also using at my desk.
15" would have been too big as I travel, and 17" definitely would have been humongous. If I were chained to my desk more, with travel only occasionally, I would definitely go for 17" or 15". But if you need to travel, but you also need to use it as your main computer, (such as when you ARE at your desk), go for the 13" Pro.
If you don't plan on having this be your main computer, and you have another Mac already, go for the Air.