I can't tell if you understand that the Lions weren't picking between Khalil Herbert or David Montgomery, they were picking between David Montgomery and Jamaal Williams. Idk why this is difficult.
Over the last 4 years Montgomery averages 1.98 ypc Before Contact, and 1.96 ypc After contact while Jamaal averages 2.34 ypc Before Contact, and 1.72 ypc After contact. What should that tell you? That Montgomery does more with worse. He isn't a home run hitter. He's a guy that gets like 10-15 yards max at a time, but so is Jamaal. Montgomery breaks a tackle on 10% of his runs, while Jamaal does on only 5%. Though I'm sure there are better stats than what PFR has, that will make this even more in favor of Montgomery
Also Montgomery is far better in the pass game with 1,240 yards on 155 catches (8 ypc) over the past 4 years while Jamaal has 929 on 135 catches (6.88 ypc) (and a 18% drop rate last season btw).
As far as the TD discrepency, Jamaal Williams had 22 Goaline Touches Montgomerys 3.