I think the On / Off numbers are best for measuring how good or bad a team's bench is, to be honest with you.
Harden has capable backups.
Russ has none.
Whether this figures into the question of how "valuable" they are is a matter of debate.
To say that Harden adds virtually no value to his team because his team doesn't drown in the minutes that he's sitting is a befuddling statement, to say the least. One might characterize it as mildly insane.
From what I understand, ESPN's RPM statistic attempts to take into account quality of teammates and opponents when looking at such on/off numbers. The reason for that is obvious -- if you have a good bench, your bench guys might be solidly in the plus against opposing bench players. That doesn't mean you could run out your bench for a full game and still win.
Harden is solidly in the + in ESPN's RPM. Russ ranks a bit higher.
I think the statistical case for either Russ or Harden is fairly obvious. To me, the best argument is what Zach Lowe stated on his podcast: You either believe that Russ's style of play comes with a team-wide cost, or you don't.
I do.
But, Russ is almost certainly going to win MVP, and good for him I suppose. He had an amazing individual season. His team was garbage in the post-season because when it came down to it, nobody on that team knew how to execute at the end of a close game unless the plan was to give Russ the ball and hope he made the shot or drew a foul.
Harden won't win MVP but he'll be playing in Round 2, and possibly Round 3.