Do you have a reason to think that portability will become in issue in the net few years? If not, the powermac is going to be noticeably more powerful. If you think you will really want a portable PPC mac, get a powerbook.
Personally I would say get the powermac. The laptops are going to be the ones that really benefit from this switch, so there will be a bigger gap in performance there. However, the powermac will probably be updated last, meaning you might be able to wait one more revision and still get PPC out of it. It seems there is a good chance we'll see a dual core G5 before the switch.
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