I had tried a Dell 12.1" M1210, Macbook and Macbook Pro all before. To me the bigger the screen/resolution the better and Macbook Pro is certainly one of the lightest 15.4" so I would recommend that. Personally I could not tell the weight difference between carrying a 13" Macbook and 15" Macbook pro.

However, 13.3" and 1280x800 is my minimum requirement that mobility could not justify any smaller spec.