definately longtubes.
X or H pipe is all in preference of sound. a H will be more deep and mellow constant rumble, and a X will have more of a agressive sound. then again, it can all change depending on the catback.
I love the spintechs side exit (roush setup) on my GT, they sound awesome and increase in noise all the way to redline. the Bassani X and Catback combo i have on my Cobra, is deep and loud till 2k rpm, then its like the exhaust dies out completely past that. i will be changing out the mufflers soon to Spintechs, see how that sounds on a 4v.