I base my decision on speed. Whichever driver seems to be keeping with the flow of the traffic is who I get behind. I try to avoid the slowpokes. I don't have the time or patience for the slow or distracted drivers.
I try not to get behind those whose head is down and locked. Friday night on E. bound 146th Street, both lanes had this. I did a slow ten count before activating the sonic warning switch on the steering wheel.