ill tell you
well, i have the same year mustang as one of my friends. He has a MAC cold air intake, and i have one i bought off ebay. They are a little different. this is how... The MAC fits better, the pipe that connects to the filter turns so that the filter goes into the fender easier. his was much simpler to install. the MAC also has better connections for the wires goig the the ingine (MAS and such.).
Mine, however, (the ebay one, came with a nicer, high flow filter, and was cheaper. It still fits and connects just fine, just is more difficult to install. you may have to buy a few connections at the hardware store so the o2 sensors will stay on nice and tight. installing the larger filter, which goes into the fenderwell and will be literally touching against your fender because its in so tight, is harder to install. However, the ebay one, sice it has a higher flow filter, sounds nicer. you can hear it sucking the air into the filter from the fender via the passenger side of the interior while you are driving. Yes, the cheaper one sound better. As far as we can tell, there is no power difference between the two, and they look pretty much exactly the same once installed.