My little story of the 13 vs 15 MBP dilema. I originally bought the new model 15, took it home, didn't open it and decided that I wanted to go smaller and save some money. I sold my 15 MBP 2 year old model a few days ago.
At first I liked the 13, it was small and fast, but the screen size just felt so small. I felt like I was on a netbook vs the majestic Macbook Pro I just sold.
So I called my local Apple store, told them I made a mistake and instead got the 15" previous model with the 9600m GT. They were VERY generous letting me return a second Macbook Pro.
I couldn't work on a 13 without feeling like it was a little task, it just all felt very cramped. I was even getting a little eyestrain headache trying to adjust. I'm sure it's not like this for ever
Now that I am back on the 15 I feel at home. I feel like I have the amazing laptop and then some that I sold, I wanted a little more meat for my money.
A 17 would probably be the best but that is too large and expensive for what I need.
Believe me I spent 2 days going back and forth and I tried to love the 13, but it didn't work out for me. It may not seem like that much extra space but it really is.....
If you are even considering a 15, I would say go for it. When you get that little 13" screen home you may regret it and whats a couple hundred dollars when you have already spent over a grand anyway.
The only thing I miss from the 13" and 15" new model is the slightly better battery, but I'm usually home with my laptop so that's really not a concern.