Just purchased a used downdraft paint booth and wanted some opinions on the best pit layout. The previous location had a standard I styled pit in the center of the booth.

"I" Style:
Pros: The floor supports the weight of the car, able to fit a rotisserie into the booth with ease, and less exhaust filter material is needed.
Cons: Overspray is drawn across the undercarriage of the car, less surface area to exhaust fumes, and there is now way to skirt the car if needed.

"H" Style:
Pros: Overspray is drawn down onto the side of the cars which is cover by succeeding passes of paint and less overspray on undercarriage.
Cons: May be more difficult to paint using a rotisserie without modifying the grates to allow the wheels to roll without getting stuck, different vehicles are different widths, and more exhaust material is needed.

Rectangle Style:
Pros: More surface area to exhaust fumes, able to paint parts off of the vehicle without covering the floors and wall in overspray.
Cons: More bracing is needed to support the weight of the car, more filters needed due to larger area, rotisseres may be hard to roll over the grates.

Are you asking for experience with each design or Opinions?

Just my Opinion
I like the idea of a Down draft booth and if I were to build one I would probably go with the I style.

BUT, There is always one.

In all the designs you posted the grates sit over a Pit in the floor.
Almost all of the Pits I have ever seen has one end or one opening on one side or the other or in the middle of Pit for air to flow to the exhaust fan.

How does the Fan see the whole Grating area as one big exhaust port so air flow is even across the whole grating system?
Without a Very large Plenum box behind or under the grates it cant.
Air will always just take the path of least resistance usually closest to the opening to the fan So it will always have uneven air flow across all the grates.

It took me a while to grasp the theory behind the Plenum Box.
It is critical to have even flow across the entire exhaust in a Paint Booth so you don't have hi flow in one area and zero flow in another.

Poorly designed Pit's
https://mcallencollisionspecialists.wor ... ecialists/

https://psistl.com/new-service-entrance ... booth-pit/

Better
https://www.youtube.com/watch?v=Xb_x_-oUDd0
But still Its a band aid for an actual Plenum.
he explains in the beginning that their was a Problem that the air flow wasn't even and shows there expensive fix for it.
This video shows the problem in the booth at about 6 min into the video
https://www.youtube.com/watch?v=IQodfZvcX7Y
The smoke is going away but More on left side of booth than right although not very clear in video it can be seen if you look for it this is the same booth before and after bandaid fix.

https://www.youtube.com/watch?v=Ifxj9FtUy8c
this one is not to bad but you can still see a difference in airflow between the two sides of the car.

It doesn't matter if the Plenum is made of steel or all built into the floor with cement, The key is to have a Large plenum Much larger than filter area so that the fan sees the whole Plenum and draws air from it, The Plenum then draws through the Filter area evenly.

The grating system becomes one large exhaust filter opening to draw air through and Not just the Path of least resistance at one end.