I vote for the Dr. Gas X-pipe. It's going to be significantly quieter than an Off Road H-pipe. As others have said, the sound quality is subjective, but there's no debating which is louder (or quieter).
If you go with a H-pipe, you're going to have a very loud car that will sound just like every other Ricky Racer out there...
Just letting you know ahead of time.
Also, no offense intended, but I'd take Flowmaster's opinion with a grain of salt.
Regards,