I don't need the extra power of the 15" (to get a marked improvement, you need to go with the upgraded GPU), and financially, couldn't justify the extra cost (and afford it right now). When it comes to screen real estate, I'm happy with 13", but to be fair, I came from a small 12" iBook screen. The portability is nice, but without having used the 15" I'm not sure how much difference it makes (I figure a bit).
I've got plans to buy a larger 24" screen for use at home.
No regrets here, in other words. But if you need the added juice, don't care about the extra size and weight, need the larger screen and above all, can afford it, go with the 15".