"I'll be using it for shooting prone, occasionally off of a bench. What is the most popular height of bipods?"
Hi there Roktoys84,
I would be using the smaller Harris bipod for exactly what you are trying to do - shoot prone. When I hunt prone, the smaller bipod is ideal. On my rifle now I have the 6-9" notched legs and for what I use it for it's great. Shooting off the bench is also great with the 6-9", I've just discovered that you also must have a rear rest too (much smaller groups

).
Cannot comment on the swivel base as I don't own one.
Cheers.