I've never played ball, but I'll give this evaluating backs things a try.
Zeke is significantly better at pass protection and picking up the blitz. He is much faster at recognizing the defensive pass rushing schemes than Pollard is and is stronger at the point of the block. With the ball in his hands, Zeke tends to pick a hole and hit it. There isn't much dancing in the backfield, and when there is, he loses yardage. He's still a big guy and strong enough to punish tacklers. He's not strong enough to consistently make the first guy miss. In the open field he no longer has much top end speed and can be caught from behind. He's still a load to be brought down...but can and will be most of the time. His style of play makes a massive difference in pass protection and just wears defenses down. Zeke has matured on and off the field. He has become the consummate teammate. Defenses know with less certainty when Zeke is in the game whether it will be a run or a pass. He really benefits from having a back that can spell him and know that there is no drop off overall with him in the game. This is especially important as the miles on Zeke's tires have been adding up.
Pollard isn't much more than a warm body at pass protection. He looks slow to recognize what the defense is doing and has little situational awareness if he isn't moving forward. When he does engage defense players as a blocker, he doesn't seem able to properly set his hips and feet to keep from getting blown off his spot. He is a liability in pass protection. With the ball in his hands, Pollard is very explosive. He has the burst and vision to change where he is going with the ball. He is much harder to defend against because he doesn't have to immediately commit to a single gap. His burst seems to make blockers take bad angles which allows him to beat single-man tackles often. In the open field he can run away from most defenders. If he gets behind the defense with the ball in his hands he is likely to score. He really benefits from Zeke absorbing the brunt of the defense's energy.
Overall, we have two great backs. We need both of them on game day as they fill two different roles. As far as the analytics? I don't have a formula. I've watched every professional play either back has ever participated in. But, I haven't broken those plays from an all-22 or any other advanced measures. So, I recognize someone on here may have a more complete understanding that better explains what I think I'm seeing. I'm also not butt-hurt if it turns out I'm wrong.