I was in pretty much the exact same situation. I'm a student and my usage is quite similar to yours, except that I also occasionally use Photoshop and Ableton Live. The 15" MBP with the high-res screen and 7200rpm HDD costs only slightly more than a maxed-out 13" MBA.
Things the Macbook Pro had going for it:
- 1680x1050 screen
- i7, and I could later upgrade to 8GB RAM and an SSD if needed
- Slightly better battery life
- It's not a brand new product so I know what I'm getting
Things the Macbook Air had going for it:
- Amazing portability. Almost half as light. I don't take my laptop to campus every day, but with the amount of books I'm usually lugging around every pound makes a difference. Also it might sound like a small thing, but with the weight restrictions on airplanes getting worse by the day, saving weight everywhere is helpful.
- The 13" screen has the same resolution as my current MBP, and a higher pixel density than even the high-res 15" screen
- Specs aren't THAT great obviously, but they're faster than my current MBP, which has been fast enough really, except...
- Really the only bottleneck on my current MBP is the Hard Drive. The only performance issues I get is when it starts paging and the HD chokes and I get to look at a beach ball for a few seconds. The SSD in the Macbook Air should help with that. The Macbook Pro wouldn't have had a SSD unless I pay another ~500.
- Hate to admit it, but the fact that the MBA looks so sexy was probably a deciding factor
Really the big question is if a C2D and 4GB RAM will cover your performance needs for the amount of time you want to keep the MBA (personally I get a new computer every ~3 years).
Another note: I highly recommend getting an external display for either laptop. Doesn't have to be a cinema display, but just any 20-24" LCD. They're so cheap now, and it makes a huge difference when writing an essay to have two screens. Also 13" vs 20" is a huge difference when watching a movie.