aquajr

I have an 06 and would never "downgrade" unless it was for a terminator. It might not be a bad idea if all you really want is boost. You can get the old GT's pretty cheap and with such low miles on your car you should get pretty decent money for it. Mods on the older Mustangs are cheaper in general too.

Bobavata

I'd wait for the '15's to come out the decide. Then you have a choice between the '14's which are a carryover from the '13's or the all new style--which I don't think will be that much of a change. Let's look at '50 thru '14 so far.

808muscle

No way I would go backwards with an 04. Just add FI to your 06, get a good tune and you will be fine. I have over two years worth of quarter mile passes with FI on my car. Thats 150+ passes. This motor loves boost!

danzcool

So, you're thinking of going from a 3v to a 2v? Not that there's anything wrong with that...

I think all the aftermarket stuff is cheaper with the older model, which is a plus, but you're giving up a drivetrain that you know the history on for one you know nothing about... that could be a good or bad thing depending on how hard you drive your cars.
Only you can make the decision, best to make it before you've sunk the money into upgrades.