My thoughts are to look for a good used '06, as I liked that model more than the '05. I think the '05 will hold resale value more because it is a first year/model, but that's not a point for me with that car really.
I might like to keep the Mustang for a long time and own other cars after it when it gets old. If of course it is affordable to do so.
1 year old cars are the best to buy, because the initial buyer took such a hit in the price. A well treated 1 year old car with low miles can be had for so much cheaper it's almost rediculous to go buy a new one unless you just can't find what you're looking for, or there are great dealer incentives on the new ones (rebates, interest rate, etc).