A little 2011 stat checking (thanks Ian):
Riddley:
87 att, 441 yds, 5.07 YPC, 1 TD, 18 FDs, 1 fum/ 0 lost, 16 0/- carries
BJGE:
181 carries, 667 yds, 3.69 YPC, 11 TDs, 39 FDs, 0 fum/ 0 lost, 36 0/- carries
BJGE, besides bringing more experience, has shown better production when rushing for that difficult 1 yard (based on the greater number of TDs). Riddley brings the bigger and more explosive gainers (based on the greater YPC). It is easy to see that each back has a role to fill (and expand based on their skills when not getting the hand off). But IF there had to be a back anointed #1 (based on the standard of producing 100 yards or more of rushing per game), I don't see an argument that makes 3.69 trump 5.07.
Fyi, Woodhead:
352 yds, 77 att, 4.57 YPC, 1 TD, 18 FDs, 0 fum/ 0 lost, 6 0/- carries