Eventually PT6 for OSX will have it's bugs worked out, and given that OS9 is pretty much history in Apples eyes, I'd recommend going for the dual processor. It will be a longer lasting machine in terms of speed/power than the single processor. Just make sure the machine you get will run OS9 and the 001. some of the new duals need some "modifications" to run OS9 and PT5.
FWIW, we recently aquired a dual 1ghz qs mac, and it is fantastic.
don't forget a dedicated audio drive(s) as well, either internal (G-4's can accomodate 3 internal ata drives w/o add'l hardware) or
FireWire.
Hope this is helpful.