If you have a screen to hook it up to, 100 percent go for the 13". I bought the 13" MBP and have no problems at all editing images, using illustrator etc. on it every day. Naturally it can get a little cramped sometimes compared to a bigger screen, but then a 15 inch will sometimes feel a bit small, as will a 17, until eventually you're attempting to carry around a 27 inch iMac - just the nature of laptops generally. Most of the time though it's no problem at all, and I can always hook it up to my monitor and get the bonus of dual screen. Also, the 15 inch just seemed a bit of an awkward size, both for carrying around with loads of other stuff, and for using on my lap. Might think 2 inches of screen doesn't make much difference, but as you said, you weren't comfortable with the 15, so that kind of defeats the point of a laptop in my opinion.
EDIT: RE: MacBook Airs. I love them, but I was put off by the quality of its spec vs. its price. As has been said, the spec has been improved and they are incredibly light and portable, but to be honest the MacBook Pros (13 inch in particular) aren't exactly overweight, so not too sure if it's really worth the extra. (Still want one though...)