Ummm the prochamber is an o/r h. The only difference between the prochamber and regular o/r h pipe is sound and power gains. I think you will see better gains with the prochamber due to its design and it sounds badass, but it does cost more. I vote prochamber all the way.
I had the prochamber with spintech prostreets and I did not like it. It has some rumble but the volume reminds me of having the stock mid-pipe. The power differences between the midpipes is minimal with a roughly stock motor. I gone through Dr. Gas X-pipe, Bassani o/r X-pipe, Prochamber, and MAC o/r H-pipe. Now I am back with the o/r H-pipe. It has the nices sound with the MAC flowpaths along with the Comp Cams.
A regular o/r h will be louder then a prochamber because the prochamber isnt exactly straight through, it has that little chamber where as the o/r h is just a straight pipe.