Let's compare apples to apples.
Since 2008 DWill has averaged 5.0 YPC, Stewart 4.7 YPC.
So Dwill is more efficient.
Very good....read past the first line lol. Stewart can make plays on his own, regularly grinding out 5 or 6 yards on each carry (I'm not referring to the YPC STAT, but to each run individually) whereas Williams benefits from the occasional big run to boost his average. They might both equate to a roughly 5.0 YPC average, but I'll take the 5 yards each time as opposed to the 7 carries for no gain followed by the 40-yarder.