I've been very impressed with the screen size of the 15". My desktop had a 24" LCD monitor so I was expecting to find this quite cramped, but it's not at all. The only time I've noticed it is when doing double-page spreads in Indesign, but you just toggle the view more often and it's all good.
My friend has a 13" and it seems fine for everyday use, but probably a bit too small for the graphic design stuff.
wow i am still undecided!
Personally, I have been sold on 13" laptops for many years - I dislike anything larger, and 13" is a perfect balance between size and power. I don't own an MBP - I have a Dell XPS M1330 - but this laptop has done me well for the three and a half years I've had it, and it's brilliantly portable. =)
I had a 17" Vaio, then a 15" Acer, now I've got a 13" MacBook Pro, and honestly, I really haven't noticed it being that much smaller.
And tbh the price for adding on 2 inches of screen is a joke.
I have a 13 inch MacBook and I couldn't really see me adding 2 inches of screen; I like the power, design, and portability of the computer as it is; I'm coming form a 11 inch netbook so the added screen estate is great.