I was almost in the same situation. All depends on when you absolutely need a car any car, and when you are expected to get your 2004.
If you are not going to get your 2004 until say June and can be assured you will be first in line for a 2005 in October I would wait. Would think some of the kinks, like the gas gauge issue, would be worked out. If you have no assurances or even esimates on a 2005 would then go with the 2004 when available. Good luck!
I don't know what kind of changes you can expect on the 2005, but I do know that you don't necessarily have to wait for ages for your Prius if you do a little digging. Some dealerships have ridiculous waiting lists, but some have Prii just lying around or being delivered unclaimed. Some dealers post on here when they have one available. I got mine by calling a dealership in Montana that had one posted on ebay - that one was spoken for, but they had another coming in a week later that has my name on it. Where I live, most dealerships are adding a premium to MSRP anyway, so paying the cost to ship my new car from another state is still cheaper than paying a markup. If you're not in a hurry, though, no harm in waiting for the 2005 - you can almost always count on improvements from year to year. Don't forget too that the tax deduction is going to decrease each year, so if you end up (heaven forbid) having to wait until the YEAR 2005 and not just the model 2005, you'll lose out a bit there too.