I just went with the 15"/1.67Ghz/128MB VRAM/Superdrive/1.5GB RAM, and im stoked. I think the 15" really is the perfect size. 17" would be nice once you get somewhere, open it up and get rolling, but getting there would seem to be a bit cumbersome. Plus if you are ever in tight spaces, like a classroom, airplane, etc. then I would think that the extra 2 inch's could make a considerable difference. Weight really isn't an issue in my mind.
Viva la 15"!!!!!!!!!!