I can answer that!
Some days I do edit Video all day or use Photoshop CS5 on a MBP 13. 90% of the time I'm using a gorgeous 24 inch 1920x1200. On those days, it doesn't really matter which MBP is it. The ONLY time the i5 or i7 will help is if I'm rending a lot of video, other wise you are taking about split seconds. On those days the i5 could reduce my time from 4 hours to 3 hours.... but I'm doing that type of work over night anyway. If I really need kick ass performance, I'd be a schmuck for using any MBP.
The 10% of the time that I'm not plugged in, I'd like something portable. I've had bigger in the past, the 13 is sooo much nicer when you are on the move. And lets face it, 13 or 15... It's still not even close to a real monitor.
Throw in the fact that the MBP 15 is 40% more money and it's a no brainer to me.