Both actually...
As I start down, I'm typically looking at the OB, more specifically a point on the OB dictated by aiming method.
If just shooting normally (simpler shots), I'm looking at and moving in the direction of my shot line, sort of an extension toward the OB. If using SEE or a similar method I employ, I'm looking at a point on the OB so that I make sure I'm moving toward it with my body and bridge hand.
In either case, about half way down or so I then focus on the CB and bring my cue into the part of the CB I intend to hit, as well as to make sure my bridge hand is placed properly etc.
I think during the whole process I have the other ball and the shot line in my peripheral vision, but I do have a distinct feeling of looking primarily at one or the other as described.
Scott