It's a bit hard to compare the two even with advanced stats. Martin and Harden were in completely different roles last year and the year before. Take a look at the top 5 scorers for each team (aside from those two).
Houston had:
Scola, Lowry, Dragic, Lee and Parsons. They averaged about 63 PPG among them and really, who on that team needs to be doubled or keyed on by a defense?
OKC had:
Durant, Westbrook, Ibaka, Fisher and Sefolosha. They averaged about 73 PPG among them. Obviously Durant and Westbrook are players that demand double teams and defensive awareness.
Martin had to carry that team and probably isn't suited for that role and I'm sure his PPG will dip on OKC, but his percentages should increase with lowered focus from opposing teams defenses.
It will be interesting to see if Harden can be the man, he's going to be paid as such, so he better produce as such. Harden benefited greatly from playing against subs and having another star player on the floor with him a lot (compared to Martin). I don't believe for a second Harden is more valuable than Westbrook. If Harden were the starter and Westbrook the 6th man, you can bet Westbrooks efficiency would jump and Harden's would fall.
As for defense, well Harden wasn't exactly lighting the world on fire defensively either (against subs). Not to mention he actually had some solid defenders around him, as opposed to Houston last year.
I would like to say I abhor Martin and find Harden overrated (some claim he is a top 3-5 SG).