do they have the same size screens? either way I'd say they would be getting the better deal
Does the mpb have applecare still left on it? Your machine should have the nvidia 8600 video card in it, which is widely reported to be problematic. If you still have applecare, game the sh#@ out of it before it expires to see if you can trade for a new one.
If not, a trade for a unibody may be a good idea to avoid the problems with your video card.
If you feel exceptionally lucky, you could always put in more ram and a larger faster hd to breathe a little new life into your older machine.