I depends upon your needs.
Firewire is better at streaming large amounts of timing-critical data.
For most purposes, you're probably going to be just fine with a MacBook with 2GB RAM, a Firewire interface and a USB hard drive (or recording to the internal drive).
If, for example, you're going to be doing mostly live tracking with 40+ channels, you're probably going to want to go with a MacBook Pro and add a second Firewire bus via an ExpressCard to keep the FW audio interface and the FW hard drive on separate busses.
Other than the expandability via Firewire 800 and the card slot, there is no real difference in audio performance between the MacBook and the MacBook Pro (beyond the expected gain due to the slightly higher clock frequencies in the Pros). If the MacBook, I would go with the 2GHz Core 2 Duo models, though, since the 4MB cache on those processors does increase audio performance.
2GB RAM should be enough for most things, though.