I had a 13" all through college and just got a 15" now. The 15" is a bit harder to transport, you need a bigger bag. But for what I do with CS5 I would rather have the extra screen space and speed of the i7 than the extra portability.
I'd say it mainly depends on what you do with your computer and where you want to do it. I'd say the weight difference doesn't matter as much to me as the size difference, the 15" just has a bigger profile when you put it in something to carry.