Well from my understanding there are several factors that determine what you get, for example
I wanted to increase my performance as much as I could at my current income while staying NA thus I opted for LT headers rather than shorty.
From what I understand is that Long tubes are more along the lines of performance where as Shortys are more for sound and little increase in performance, now I could be wrong so don't hold it against me. Also the money you are willing to spend is another BIG factor, if you have the money and want as much performance as possible why not go for Long tubes, if you don't but still want performance you can opt for the Shortys or wait till you have enough for the Long tubes.
If you get Long tubes you are gonna need a Shorty Xpipe and with this you have 2 options, the catted one and the off road one. Both are good but depending on the laws and regulations from where your at and if they do emission testing should determine which one to get. From my knowledge we do emissions where I'm at so I went and got the Catted X pipe, but there are people who get both and just switch out the catted one for the catless after they have done emissions. The catless X pipe will give more performance and it will also make the car much more raspy and ricey sounding, the catted one helps bring down some of that raspiness. Another reason why I opted for the Catted version.
Now if you go for the Shorty headers you do not need a Shorty Xpipe instead the Shorty headers use the factory x-h pipe hybrid that ford put on the cars. You can also purchase an H pipe or X pipe to help even the flow with the Shortys if you wish but you don't have to, but if you have the money then why not.
Another factor would be sound, you have to listen to sound clips online but they will never match the sound, the best way to see if that is the note you want to hit is to buy some exhaust and put it on. Everyone has different tastes some may like that loudness of headers, where as some don't, it all depends on you and what you're looking for.
Another note is that you will need a tune for the headers, I'm not sure about the Shorty headers, but I know you will need one for the Long tubes, somebody with Shortys can chime in and tell you if they needed a tune or not.
Sorry for the long post.
2011 V6 Black Mustang Automatic: Magnaflow Competition Series Cat-Back Exhaust, Airaid Cold Air Intake, Raxiom Smoked Projector Headlights, Raxiom Smoked Aero Tail Lights, 93H-R3.1 Bama tune, BBK Long Tube Headers, BBK Catted X Pipe