um...
This is not an A or B question.
Depends on what you are editing on now. What system? What kind of content are you editing?
Editing video on a laptop is not ideal yet unless you HAVE to be away from your main system. Offline and DV will be okay, but it is not ideal as a primary system. If you need to render anything, do any titling, or work with any uncompressed media you will want a desktop based system.
FCP 4 runs really well on a Dual 1.25 G4 so maybe you could get one of those, save some cash, and then get a powerbook in 6 months or so.