I would say go for the 17" with the 1920x1200 resolution.
I'm a graphic design student and had a 15" PowerBook for about 3 years. I finally came to the point were I needed a more powerful computer with a better screen. Just last month, I bought the top-of-the-line MacBook Pro at my local Apple Store. The specs are: 17" 1920x1200 high-res matte screen, 160 GB 7200rpm hard drive, and 2 GB RAM.
This machine flies and is a pleasure to use with its beautiful screen. The added resolution is a real asset for Photoshop, Illustrator, InDesign, Soundtrack, and Final Cut.
The extra size is great for recreation, too. I really love watching movies on this laptop. The FrontRow remote is a nice bonus for DVDs as well.
I used to bring the 15" PowerBook with me to class all of the time, and now I carry the 17" around. Honestly, there isn't much of a difference to me. IMO, it doesn't matter if I'm carrying 5lbs or 6lbs. What matters to me is that I'm using a super-powerful Mac with the best laptop screen available for my work.
I hope this helps somewhat. Buy whatever you want, but if you like the 17" I say go for it.;D