Well, let's not just be all emotional about it. Numbers are good.
907 > 556 Advantage D'Angelo
4.5 ypc < 4.9 ypc Advantage Bell
16 games played > 6 games played Advantage D'Angelo
56.7 < 92.7 Advantage BELL (note the BIG advantage here...) See, I can be objective...
3 fumbles > 0 fumbles Advantage Bell
Based on the above, I think it's safe to say that Bell is generally a better overall option than Williams. The yards per game is a glaring stat that's huge. Of course I didn't put up the carries per game stat. Bell was getting at minimum 6 or so more carries per game than D'Angelo and Williams was almost completely unused in the weeks before Bell was was injured. He played during the suspension, was relegated to the bench excepting a play or two here and there and then did virtually nothing until Bell was injured.
Nonetheless, I don't think any reasonable argument can be made that statistically Bell is not superior to Williams before Week 6 or 7. However...
After week 7, Bells production fell dramatically. In fact it could be argued that without Williams the running back situation would have become dire. Bell may as well have been inactive for as low as his statistical output was after November 2nd. Oh wait... He was inactive. So it seems Williams was a significant upgrade from the point on. You could even say that D'Angelo Williams saved the entire season by giving the Steelers a semblance of a running game. I would say that ensuring that Williams remains as the #2 and finishes out the 2nd year of his contract is a major priority this year.