Go X-PIPE, BASSANI X FTWWWWWW!!!!!! The H Will give you more of a deeper sound, but you will see more power gains throughout with the X PIPE. It also depends on your catback. If your gonna go American Thunder go H Pipe, it will fit.
What you should do is Sell that American Thunder and go
Borla or Bassani.
I have a Bassani X Pipe to
Borla catback. I didnt want mine to be too loud, cause theres crazy amounts of cops in NY. But when i get on it, it sounds like a beast, nascar style, not like just a LOUD POS. Its a more aggressive tone.
My friend has the complete opposite, Mac Flowpath catbacks to Mac Prochamber, and I like that too, its more personal preference. And also how much money you have to work with.