In 2012, he had 15 carries for 105 yards and a TD against the Raiders. Other than that, it's a small sample size. Ryan Williams in preseason? The game he got the most carries, he had 47 yards on 12 carries and averaged 3.9 YPC, nothing remarkable.
And hell yeah I'm going to take how a player performs in regular season games over preseason. I don't even put much stock in the performance of what Dunbar did. Players often prove themselves in TC not in preseason.
I'll leave it up to the coaches to pick between Dunbar and Williams if the hypothetical situation occurs that Randle gets released. But, as of now, I'd choose Dunbar over Williams if we had to. But, as I said, I'd draft a replacement. This isn't even just for Dunbar and Williams, I will be looking for competition for McFadden and Randle as well.