I'm sorry but this shouldn't even be a discussion. Put Bettis in against defenses today and he would shred them even more than he did in his era. You spread the defense out and watch Bettis truck linebacker after linebacker. Let's see how Bell would do against 9 in the box that Bettis would regularly face in the 4th quarter ... and he would still grind out 1st down after 1st down. I don't care how good your line is that is a tough task.
In today's game there is always the threat of a pass which works in the running backs favor. Just take a look at New England's ground game to prove that. In the Bettis era it would be "hi diddle diddle Bettis up the middle" and there wasn't a damn thing the defense could do to stop him. Then throw in the fact that Bettis was a good receiver ... he would love to catch a screen pass with a little space ... and this discussion shouldn't even take place.
I hope in 4 or 5 years this discussion will be relevant because that would mean Bell has turned out to be a great back but I don't think we can say that right now.
I agree 100%... watching that Bettis Tribute video I see LB's and DB's bouncing off Bettis like he was an over sized beach ball.