That's pretty vague and hard to define. I think you'd be hard pressed to make that case. I think it simply reflects a personal preference. Rainey was much more likely to get injured, get a penalty, or fumble, or get arrested. I enjoyed Rainey and loved that we had them both. I think it's hard to claim that Rainey was "more of a running back." You'd need to define that one.
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As to the claim that Rainey was faster at the 40 and Demps faster at the 100, I found this where Rainey's personal best was recorded at a college track meet. Rainey's PB is 6.7 in the 60 and Demps beat him that day with a 6.61. Demps' PB according to Wikipedia is 6.52.
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