Hate to say it, but I'm sort of confused here. The 12" Powerbook is no less out of date than a 15" one; it is in fact more limited, with its lower RAM ceiling compared to a (fully working) 15" and its cramped little periscope 1024x768 screen. In response to "Smaller then the 15" I was looking at, so could carrying it about with me", the 12" is 500 grams lighter than the 15", IE, 2.1 vs. 2.6kg. I guess that's 20%, but does that really make that much difference? And finally, last I looked (granted I haven't been keeping track) 12" PBs were no cheaper than 15" ones; in fact they seemed to be somewhat more expensive. (Checking Craigslist today I guess I'd say it's about a tie overall, but a 12" is the second most expensive listing after a 17".)
It's going to suck in all the ways listed above, plus it's going to be more cramped than the bigger one. (Granted you may be used to it, it does have the same screen resolution as a Pismo.)
Can't help you there, of course.
As I said before, if you want the Powerbook (in whatever size) for the nostalgia value (/slash/ to run some PPC software that's really important to you and absolutely 1: can't be updated, and 2: won't work acceptably under Snow Leopard in emulation) totally go ahead and go for it. But you really shouldn't be comparing it to a MacBook *at all* in that case. They're two different animals.