If your putting a new exhaust system on your car, I would highly recommend you get it professionally done unless you have the correct tools to do it yourself. If you mess up any threads at all you might as well say bye bye to your headers and hello exhaust leak.
I have a BBK X pipe with No cats on my car and I love them, very deep and throaty tone.
If you are going to put money into a X pipe, dont get flow masters. They rust out in 2 or 3 years, get the good stuff, BBK, Pypes, Bassani, ect.
I have only heard the BBK (on my car no cats) and Bassani X Pipe No Cats (my dads car) If you want a exotic sound that makes you really stand out from other stangs go with the Bassani X pipe and keep your stock headers. My dad's stang got a little quiter after he installed BBK Headers on it.
Hope that helps a little and good luck!