12" vs. 15"

So, I've decided I'm going to get the Powerbook G4 so I can be part of the cool-kid mac switch revolution, plus the added bonus of website testing...really just so I can finally record my album using Garage Band.

What I'm debating now is whether to go with the 12" or 15".

I've been enjoying my eBay experience - watching some auctions go through, getting a sense of the marketplace. It's really an incredible tool for a motivated buyer.

Here's what I want: at least 1ghz, 512MB Ram, 60GB HD, iLife.

Which one should I get?
How much should I pay?

Quick, tell me before I buy!