I would definitely go for the PowerBook.
Is the G4 slower than the P.M? Probably--but that doesn't make it "slow," just "not as fast." People edit video productions on PowerBooks. I do 3D animation and run my whole graphics business on mine, and mine is just 1.25 Ghz.
Is the productivity, security, and privacy of Mac OS X worth the CPU difference? No question. What matters about a computer is what it DOES for you--not Mhz numbers. Unless, of course, you are building a render farm out of laptops
Coincidentally, someone here just posted a review (different size screen) that you might find helpful--it's by a long time Windows users getting a PowerBook:
http://www.notebookreview.com/default.asp?newsID=2400
Having used both Windows and OS X, there is no way that Windows is an acceptable alternative. Once you are used to Exposé and Spotlight, you'll hardly tolerate anything less. Go with Tiger, whatever the CPU, and give yourself a treat
Also, the extra res of that Pentium machine would concern me: that many pixels in that small a space "sounds nice" but can really be hard on the eyes during long use. That's subjective, and some people do find those tiny menus usable. Your call--but an external display makes the issue moot when you're at home anyway.
As for RAM, both machines have plenty. 512 isn't ideal for your purposes, so I agree, get more.
Now, WHEN to buy?
There MIGHT be new PowerBook G4s this year, or there might be none until the Intel Pentium M (Yonah?) PowerBooks next year. If you wait for next year, they could come as soon as January (and I hope they do) but they could come as late as June or July 2006.
But I see very little chance of new PowerBooks by July of this year. Maybe... but unlikely. The ones out now are still fairly new. And there will be NO G5 PowerBooks, that's now pretty certain. G5s are too hot.
So if you need something by July, I suggest getting one NOW--it's the best laptop on the planet for most people, I'm convinced. And if you need more speed, start saving for some insanely fast Intel PowerMac in 2007 or 2008

Don't worry about buying a PowerPC now: they'll be supported for years. The next Mac OS (Leopard) will run on them, and so will future apps--at full speed--thanks to "universal binaries." In fact, early adopters of Intel PowerBooks might have LESS software choice than a G4 has now, depending on how fast developers port their apps to Intel.
Try-before-you-buy is a good idea, but I doubt you'll be allowed to return a working PB. If you can get to an Apple Store, ask them if they can put Pro Tools on a display PB to try. You never know. Failing that, see if you can try some other audio apps (they might have Soundtrack already installed). Then see reviews to make sure Pro Tools isn't much slower than whatever you tested. Or ask for reports about Pro Tools on a 1.5 G4, both here and at the MacNN.com forums (two big Mac forums).
Also, compare the 15" and the 12" screens at the store. The 15" screen is much better/brighter. I got the 15" and I like it, but I actually wish I had the 12" for portability! It's SO tiny and sleek. Oh, well. (I'd use an external display while at home, so the built-in screen wouldn't matter. If you don't have an external screen, the 15" PB is worth it probably.)
Lastly, look for discounts: Apple student/teacher pricing is great, and Amazon has nice rebates on Macs. Other resellers like macconnection.com etc. bundle free stuff (sometimes RAM) in order to compete with each other. Even Apple has specials and sales sometimes. And you can always get a free color printer (sometimes printer/scanner) with any Mac--see store.apple.com.
Good luck! Let us know how you fare.