I'm actually surprised to see a majority of the responses favor the 13" rather than being more balanced. Like many, I debated with between the two and what version, but in the end I went with a refurb late '08 15" with the dedicated graphics. My wife uses a 13" white macbook, which I used on occasion and my work provided a 15" Latitude replaced by a more recent 14" Latitude. I'm also testing out a 17" HP business laptop with 1600x1200 reso. So needless to say, I've tried it all, including a tiny Dell Mini 9.
They all have their pros and cons and it really comes down to your usage. I'm a developer, so screen resolution is important to me and my MBP is mainly for use around the house dev and also video editing. My toddlers are always on the move, so I can't really be tied down to a desktop. The compromise was the 15" but I would've loved the 15" to have 1920x1200 reso or even if a 14" MBP was available with 1440x900 reso like the 15". In addition, I do plan on getting a Apple "tablet" when it comes out, and I figured, I would prefer a bigger screen size difference. I could see the possibility of me replacing a 13" with a tablet (due to size and specs) but couldn't do so with a 15" due to it's size and specs.
So, although I have much respect for the 13", I'll add a +1 for the 15" tally. BTW, my 15" refurb was $1299, which was so close to the 13" prices.