The tradeoffs between the 12" and 15" are basically performance, size, and price.
From the performance standpoint, there is almost no noticeable difference between the lower model 15" and the 12". Both run at the same speed with the same frontside bus and either can be upgraded with a faster hard drive. The NVIDIA graphics card in the 12" vs. the ATI in the 15" is rather significant, but only if you plan on doing a great deal of graphic design or playing the latest and greatest games. The gigabit ethernet port, FW 800, or PCI slot are things you won't miss in the present, but probably will down the road as you upgrade your Powerbook. The 12" smaller size limits room for expansion and next generation ports.
Sizewise, the 12' is much easier to lug around. Trust me. The 15" is definitely manageable and much lighter than comparable Windows machines, but if you place a premium on portability, the 12" will serve you better. The obvious tradeoff is screen real estate. The 15" screen is quite sufficient as a desktop replacement for almost anything you'd need. The 12" screen hasn't been a problem for me, and I use the smaller powerbook as my primary computer nowadays. I usually only look at one window at a time and with Expose, I can easily manage multiple windows in the background. However, if your eyes are easily strained, or you aren't accustomed to managing windows on a smaller screen, the 12" might be a little frustrating for long-term use as a primary machine.
If price isn't an issue, then your choice is really just about personal preference. If it is, though, you might consider what else you'll need to go with your new powerbook. For the money you'd save with the smaller powerbook, you could pick up some software, a bluetooth mouse, new case, airport station, etc. Conversely, when you factor in longevity, the 15" might be the better buy because it might last up to a year longer with a pc card processor upgrade and a fast firewire 800 hard drive.
That said, I'm a full-time law student and carry the 12" back and forth between home and campus daily. Commuting by bike with a load of books and my computer, the smaller powerbook is definitely more mobile. For anyone in a similar circumstance, I would recommend the same.