Lol, ok there are no MEANINGFUL changes from 03 to 04... As in the OP wanted to know if one was better than the other, no they are not, they are the same powertrain with minor cosmetics like pretty much any year which would go without saying...
As for the blue stripe, that's what I get for listening to the rumor mill. Good to know. One good thing to do would be to pull a spark plug and take a feeler gauge to see how many threads are in those heads which is the big one. Most 03s had the 4 thread plug holes. 04s were revised, this I do know. I also have part numbers somewhere which you could just look at vs blue paint lol.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.