I'd definitely go the PowerBook route, but that's because I have one.
Apple has taken us music types under consideration and has APIs built into the OS (Core Audio / Core MIDI) for software developers to use. Because of this, you don't have to worry about if your card has ASIO drivers or WDM drivers or whatever because if it's Mac compatible, it'll come with Core Audio / MIDI drivers and will be available in all of your software.
Same thing goes with software effects, Audio Units is part of the Core Audio spec.
Beyond that, the PowerBook comes with GarageBand so you can start recording tunes the second you get your PowerBook turned on. While this is a simpler program (can only record one stereo track at a time) it will get you involved with audio creation / editing lickety split. It took me about 10 minutes to know what the heck I was doing. While you are stuck recording one track at a time, you can add as many tracks as the CPU can handle, and they can be audio (line in / mic) or MIDI generated. MIDI editing is a bit painful, but the ability is there, so if you play a sour note, you can go in and change it to what it should've been without re-recording the whole track.
When you're ready to move beyond GarageBand, you'll need a multitrack I/O box (either Firewire (IEEE 1394) or a PCMCIA card, I wouldn't use USB unless you aren't recording over 2 tracks at once) like the M-Audio or Edirol devices. I'm positive the M-Audio units come with software (Reason Adapted, Live LE, stuff like that) but I'm not sure if the Edirol devices come with software.
If not, there are a bunch of software options. You can get Logic Express for $299 (149 if you're a student, you can also get a discount on the PowerBook if you're a student as well), Bias Peak or Deck, Ableton Live, and more.
Beyond audio, there's video editing (in case you want to make a music video) with iMovie which shouldn't take you more than 10 - 20 minutes to figure out how to use well. I kid you not, Apple's software is quite intuitive.
Not only that, but everything 'just works' on the Mac. Plus you don't have to worry about the latest virus / worm / exploit floating about attacking Windows computers.
Also, the email program has built in SPAM filters that learn as you identify junk mail that slipped past the filter. Honestly, about 1 email a week gets past the SPAM filter so I find it relaxing to not have to prune 10's of SPAM messages every day.
It took me about 3-4 months of using my first Mac to get it. The OS slips out of your way and lets you do your work. It wasn't until I hadn't had any driver issues or whatever for some time to realize that I wasn't fighting with my computer anymore, I was using it.
I'm sure other Mac users on this board can enlighten you about working with audio + general computing on the Mac, but the above is my opinion, coming from a recent CS graduate who's been using PCs since '93 and moved into the Apple universe in 2001.