My experience with Powerbooks/iBooks in this range is that there can be little between them as they can tend to be a delicate balance of hardware variance and software preference.
I have a 17" DLSD and last year had the 15" too at the same time - even though they benchmark identically, the 17" seems miles ahead.
I have a 1.33 12" Powerbook, last year I had the 1.5 version too - the 1.33 always seemed faster, despite the illogic of that.
I don't think there's any guarantee with this class of hardware now, after all, despite what I've said about the 17" DLSD, there's users on the forum that aren't happy with theirs.
I'd consider an Intel before shelling out or just bide your time for a bargain....