I wouldn't say it's a stupid question. It really depends on whether you want more power or more portability.
The iMac G5 will, obviously, be faster than the Pbook, but decidedly less portable (read: not portable at all). Weigh which means more to you and decide with that. You don't mention any specifics on the Pbook, so it's hard to say whether the G5 might be more expensive in the end or not (it would all be speculation anyway). In the end I think it all depends on whether you have a desire for portability or not. Hope that helps!