Any painted steel will be slippery when wet.
Depending on how level the ground is where you're parked, a rectangular tube could have you sliding off easier than a round tube (if it's facing downhill).
A round tube always has one, and only one point of contact when you put a flat object like a shoe on it.
I still can't figure out why anyone ever wants to stand on a rock slider. They are body protection, not step rails.
If you need something to stand on, open the door.