From where are you travelling?? That will make a big difference in where you go. I think from most places, Bali would be easier to get to than Fiji, but there are direct flights from Honolulu. I think you would need a LOT of frequent flyer miles to get a free ticket to either location. And, again, depending on where you are now, a week isn't much time; you could spend 4 days just getting there and back.
As for diving, did a few dives on Bali several years ago. It was OK. Never been to Fiji, but from I hear, I would think it has the better diving.