I'm no RB expert, so you'd do better to
ask @bshaw28 questions concerning the RB.
But I would say, Parrish & Martinez both have very good vision, which will gain them a lot more yards on average than most RB's, simply because they see the hole/running lanes so much faster & can get you an easy 5-6yds on an given carry.
The difference, is mainly style of running. Parrish wasn't a tough in between the tackles style of runner, he's more finesse & had his best runs when he was able to bounce outside after making an initial cut. Martinez is a downhill bulldozer who seeks out contact. He can bounce outside as well, but he's primarily looking to get to the 2nd level of the Defense, lower his shoulder & run through your chest.
Neither one is all that fast, I'd say they're both in the mid 4.5- 4.6 range, but for RB's speed isn't necessarily as important as it is at CB & WR. It certainly helps & you'd prefer a fast RB over a slow one, but there's plenty of RB's who aren't really blazers that are still very productive & effective.
