don't know why they prefer one over the other.
in my opinion, the best way to hunt rubs, is to first figure out which way the buck is going. for instance, say the buck is comin from his beddin area, and the beddin area is in the north corner of the woods your huntin. if the rubs are on the north side of the tree, then the buck is comin from his bed to a feedin area. if the rubs are on the south side of the tree, then he's headed back to bed up.
I would follow the rubline back toward the bed area, and try to find a funnel opf some type( a creek crossin, a fence line) set up on the down wind side of the rub line at the funnel. and check the rubline often, for fresh rubs. that's when you want to be sure and hunt that rubline. hope that helped.

daytona 500 winner- Ward Burton
go Cat-22