Yes good question.

It is impossible to answer because it requires the ability to see into the future. Since they are offering no meaningful discount on the '04 there is no reason to be in a big hurry. The 2005 will be worth more in the long run when you go to trade. For example a 2001 Insight is worth about $1000 more than a 2000 according to Edmunds. If you plan on keeping the car a long time this will matter less and less over time. If you only plan on keeping it a few years it is a pretty big deal.

I suspect that in the future Hybrids will be much more popular as more people learn about them and as gas goes up. This will make the current models harder to find and will make them cost more. On the other hand as more companies bring hybrids to market and as the technology impoves the current models will tend to be worth less.

In 2006 diesel in the US is going to be a lot cleaner and by then a diesel hybrid might become available. Honda already sells a very cool diesel Accord hybrid in europe.

Personally I've got my money down on a 2005 magneisum MT HCH because it is an '05 and is the color I wanted and has the MT I wanted. I passed on the '04 white CVT because it is an '04, and is white and is a CVT. The MT is less expensive so I'll end up paying less for the 2005. I figure I can alway trade in 3 years or so for whatever is available in 2008 and beyond. If the peak oil folks are correct gas is going to get darn expensive and those of us with hybrids will care less.