15" for me.
Had a 13" Blackbook, told myself I would get a 15" MBP the next time around because I wanted the aluminum casing, keyboard and dedicated graphics...
Then along came the uMB, suckered me in with it's somewhat fulfillment of the above outlined aspects. In less than I year I can to find that I should have just bought the 15"-- so I sold it and here I am.
I feel that my next Mac could be a 17" MBP (in a couple of years)... Though maybe I would just get an iMac for a higher resolution experience at home. The res on the 15" isn't so bad. Especially when compared to the 13".
You notice the resolution difference a lot from the 13" to the 15" do you?
Hard for me to tell my 15" MBP is great .... but my 13" MacBook Air also great, but the Air has a very nice screen (less glossy), compared to the MBP's......