I have had both DMH and QTP on my Z28 and DMH is a better product. I have mine installed after the headers and before the cats. Theres alot of places that wont pass that setup when it comes inspection time so make sure you have a cool dude doing it
Mine are cheap manual summit brand cutouts welded in right after collector and it is so loud it's funny.
Reason i got the manual (besides being 1/4 of the price) is because my kids (ages 4 and 19 months) ride in my truck 90% of the time and i knew i wouldn't be driving around with an open exhuast with them in there but i can still use it for the track when i want to.
If your lowered or going to be lowered get auto's if not manuals aren't that bad.