The coaches like Randle more because he is better at pass protection. A backup RB is obviously going to be a dropoff in running production from the starter - but if your #1 back goes out, you just can't put a guy out there down after down who doesn't pass protect well.
The problem we have with these backs is similar to problems we've had in the past. Our #1 is just the best in terms of running, receiving, and pass blocking of the group. That means we want him on the field all the time.
The reason we like Randle is that he can actually block at a similar level to Murray. If we go with Randle it is all about how you keep Murray healthy - by trying to cut out a few snaps every game. Williams is the guy you want if Murray goes down, Randle is the guy you want playing 7-8 snaps a game to give Murray a breather without getting Romo killed.