The 12" PowerBook basically is a dual-head aluminum G4 iBook that costs a few hundred more. The actual iBook may inherit the G4 once the PowerBooks inherit the PPC 970 (probably some time in 2004). It also may simply utilize faster G3s instead, and not move to the G4 at all.
A 700 or 800MHz iBook should more than cut it for what you're using it for in Logic. The G4 will give you a little more headroom, but AltiVec aside, it's more or less even with the Sahara G3. Both the PowerBook and the iBook should hold their value equally well, although the PowerBook will of course be worth more in any given length of time because it cost more to begin with. The G4 is great for the types of things that use AltiVec, like real-time effects, but for plain old 5-track playback and recording with MIDI synthesis on the side, AltiVec won't make too much difference.
I have a 550MHz G4 PowerBook that does the type of audio work you describe with no hiccups (although with Cubase 5.1 in OS 9). Real-world CPU speed is probably only about 65% of an 800MHz G3 (although AltiVec puts it ahead in some areas). The 12" PowerBook is better than the 800MHz iBook, but sometimes it's nice to save $500.