13"
reasons:
1) i want my laptop to be small and portable, i have used a friends 17" and its just to big for me to take to classes or take into a small cafe and the like. I mean you can do it it's just a tad more effort. My friend mainly uses his in a manner akin to a desktop only one that you can move round the house.
2) i don't really do much that needs a lot of screen real estate. Mainly word processing essays, rough demo's on garageband, watching a dvd in my room, itunes. 'Spaces' and 'expose' are such a cool way of getting round screen clutter on the 13".
3) The 15" i haven't used, and as i don't have much money i had to settle for the base 2.26 13" MBP. I suspect the 15 is probs the best in terms of fairly high performance and portability.
I am happy with my 13" mbp and i got good educational discount and the old 15% VAT rate in the uk last year, which made it quite a reasonably priced machine (still pretty f'n expensive for me tho, lol). Have to stop myself from looking at the threads laying in to the base model though as i start to wish I could have waited for the refresh. Unfortunately my dissertation year would have been a dumb time to not have a laptop.
:sigh: god-damn technology and its ever changing-ness.