Well, the only ones that will fit your X-pipe are going to be shorties. Shorties are the same length as your stock headers, but are way less restrictive. Long tubes are full length headers and they go all the way down to underneath the car, you will then need a different X-Pipe or have to have yours modified to fit them. Bassani has some mid length headers out now which is the way I'm going to go. You get great gains with them without having to pull a header to do a clutch.

As for the differences in both, basically you will get more low end torque from LT's, but you may sacrifice some upper RPM HP. Hope this helps.