The issue i see with Penny is that he seems more interested in hitting the big play on every carry than he does just churning out drive sustaining yards and taking the big hitter when it comes. i'm not sure if its because he's been used to being a #1 and is trying to make the most of his opportunities by cashing in big to justify being elevated above Carson or what. Just seems like he's always dancing in the backfield, changing direction, or looking to bounce it outside rather than just hitting the hole and seeing what he can get on the other side. And that type of running style is a hard thing to build into an offense that's trying to maintain rhythm and stack first downs because its often feast of famine.
Id be interested to see what his YPC are if you take out runs over 15 yards.
Carson may not get you the 50 yard pop, but he can hit 4 to 10 yards a pop all day. More workman like, but also much more reliable. That's what makes the guy hard to pull.
You can get to 5 YPC buy going 0,0,0,2,0,3,30. That's Penny.
Or
You can get there by going 3,5,2,7,7,4,7. That's Carson.
Chances are your offense is going to be more effective in scenario 2 than scenario 1. But everybody remembers the 30 yard carry and forgets the drive stalling no gainers, or 1 yard duds.