Frankly I would put both in the top 5 but neither at #1 or even 2. I think the top 2 are Brown and Payton. After that there are mutiple ways to go. If you value longetivity, Emmitt is very hard to ignore. People talk about his champion years, but people overlook the fact that he had 1,200 yards+ 9 out of 10 seasons at one point and 11 consecutive 1000 yard seasons.
Sanders pure production cant be ignored. Even for his detractors (including) he was the most prolific back in terms of production.
In terms of a shorter term production, Dickerson and Campbell deserve mention.
In terms of the ability to dictate a game, Faulk is a real wildcard. Given his ability as both a runner and the ability to play any WR slot, had he not been injury he may have gone down as the best back ever. However full seasons out of him were rare
Personally I rank them 1) Brown 2) Payton 3)Smith 4) Sanders 5) Dickerson