I've installed and used both, I prefer the dual pit design with a 2' X 2' H connector (I've boxed in the H connector and used a culvert design, both work) in the center versus the single pit. I seem to get better, more even air flow using the dual pit and with the single pit design it seemed a lot more difficult to balance the booth as far as turbulence goes. With the single pit it seemed I could get rid of air turbulence in 1 or 2 corners and then they would appear in the other corners of the booth. The booth's with the dual pit with respect to balancing, it seemed that the dual pit version stayed balanced longer as well. I've used the smoke test to see how fast the booth clears out, they both seem to be equal to a degree but, as I mentioned, I would seem to get swirls of lingering smoke in 1 or 2 corners and the H pit minimized this problem. The last 3 booths I installed all went with the H pit design and had excellent results with happy customers and clean paint jobs.
It seems people get better, more even exhaust flow and back pressure on a car with an H pipe, why not a paint booth.
Hope this helps