I'd say go for the 15" for the extra screen real estate. You dont necessarily need it for reason, which is fairly fixed, but if you ReWire it to Logic or Live, you'll be grateful sooner or later.
Unless of course you plan on running an external monitor, which the the size becomes less of an issue. If ultra portability is a concern obviously the 12" would be the way to go, having said that the 15" is still perfectly portable (I carry it to work everyday so I can do some wonderful things during my lunch break).
As emotion mentioned, the other advantage of the 15" is the PCMCIA slot. While this wont be much of an issue when you are using it at home (the core audio drivers with OSX are low latency), should you decide to take your tunes on the road, or if you decide to get some good quality monitors (the speaker kind, not the visual kind) you'll maybe want something like the Indigo. The advantage of this is that it frees up your Firewire ports for External HD's and the like.
Another note on portability is the keyboard you are going to use. I recently bought an M-Audio O2, which I think perfectly compliments my Powerbook. Its only a little wider than the PB, and a touch deeper (excluding the protruding knobs of course), and once you get used to the pitch bend and mod wheels, and the reduce travel its great.
Hope I havent rambled on too long!
:EDIT: forgot to actually respond to the previous post! Yeah Reason runs a tad slower (havent tried the 3.0.3 update yet), but it still does everything you want, and you get the bonus of OSX!
I think its safe to assume you can spend the extra money and not have a new Powerbook come out soon after. Even if there is a new one, there'll only be a slight bump in speed. Its highly unlikely there will be a G5 Powerbook before the switch to Intel. Either way you'll still end up with an awesome machine. I bought my Powerbook recently, and I really dont care if a new one comes out tomorrow - mine does everything I want it to, and I couldnt be without it while waiting for a new one! I know someone who still uses his 1 GHz 512 MB Ram Powerbook with Reason/Cubase SX/Live.