Ceiling Rib Furring Strips

[Curious Slug]

So, I'm planning on using 1x2 furring strips on the walls and ceiling ribs. it looks like people go one of two ways for the location of ceiling rib furring strips - either directly under the rib or to the side of it.

Is the only advantage to installing strips on the side that you don't lose head height? I can think of a few disadvantages, just want to make sure I'm not missing something.

We are insulating with sprayfoam and we have some head height to play with so I'm tempted to go with underneath the ribs i.e. 'into' the living space of the bus. I figure this will allow me extra depth for spray foam (hoping to get 2" in there) and reduce the chance of heat transfer between the final ceiling and the ceiling ribs.

I'll be using ripped foam board as a thermal break between the furring strips and the ribs. I'll also be counter-sinking the screws so they don't touch the final ceiling finish (TBC but probably T&G).