X pipes are the best but you only see real performance gains in non-emissions cars. I put them on my 69 Mach 1 and difference in my throttle response was unbelievable.
If you are considering installation of an X-pipe or H-pipe on a late-model V6, they are not necessary since no two cylinders on the same side side of the block fire in sequence. Running true dual exhaust by welding straight pipes after the stock cats (if you have them) is fine, by either having them fabricated by a muffler shop or by using one of the aftermarket conversion kits. You can then use a GT takeoff exhaust or any aftermarket GT catback that fits your model year.