i have a 17", and my two best friends have a 15" MBP (same specs only matte) and 13" Macbook repsectively. Obviously, i'm partial to my 17", got a discount for a timbuk2 XL commuter bag, fits the 17" perfectly!!!!
great bag, the 17" honestly is just as portable as my friends 15", really no difference at all... for me it's not about extra screen size, it's about extra screen resolution... i like the extra res for Final Cut, it really is nice.
all 3 computers are solid choices though.