Congrats on your recent 12" purchase Rapps. I think you made the right choice - you went with a mac

. As for a debate over the 12" v. the 15", I think you would have been more than satisfied with either decision - but here's why I like the 12.
I am currently working on my master of public policy at USC and recently purchased a 12 for my computing needs. I have friends in law school, and their computing usage is virtually the same as mine - lots of research and writting with some fun media/music usage in your spare time. Any of the PBs can more than handle anything we could possibly throw at them while doing simple grad school research or our light personal usage, which, I believe, rules out any real hardware performance concerns you might have between the agile 12 and the mightier 15.
This means that there are only two real issues left for us primarily academic users to debate: 1) cost and 2) screen size vs. portability. Whereas normally it is the other way around, strangely enough, in this instance, working through the second issue appears to define the terms of the first. If you wanted to simply make one purchase to do all and be all that you could possibly want in a computer, the 15 is your item. It is sufficiently portable to go anywhere, and its screen large and sexy enough to satisfy both your aesthetic and practical usage desires. However, it seems to me that your potential purchase of a large flat-panel monitor, negates the primary advantage of the 15. And if the issue of the 15's greater viewing area has been largely mitigated, then the only real determining factors in your purchase are portability and price. The disparity in size and weight between the two models is not large, but certainly the 12 enjoys an advantage sufficient to make a difference to you. Moreover, the 12's cost savings will allow you to get all of the upgrades you desire and still put unspent monies toward the purchase of one of them new 20" beauts. The 12 seems, at least to me, like the smart purchase here.
On a purely personal note, I am loving the "smallness" of my 12. I feel like I can take it anywhere at just about anytime and enjoy doing so. My old 15" Compaq always felt like a burden to pack up, whereas everytime I plop the 12 in my bag I get a rush of joy. Also, there's something sexy about pulling out such a potent machine, wrapped in such a small and sleek package.
Anyway, I hope my comments have helped alleviate any "buyer's remorse" you have whenever you see those wonderful wide-screens on the 15s. (Just remember that the engy is reciprocated via your 1.1 pound and 3" diagonal advantage.) Good luck in school, and enjoy!
- JP