It really is tough to call but you have to think there was some real reason Merab desperately didn't want this fight.
Umar elluded to the fact that Merab's wrestling is lower level and he's only looked so good because he hasn't fought any Sambo elites. Umar also has far superior striking, submission and finishing ability in general.
Then again Merab's offense seems weak in terms of finishing but I've never seen anyone push a pace like him and he's beaten some very legit fighters.
I think I'll go with oddsmakers where Umar is the heavy favourite. I think he'll land a lot more shots and outpoint him in a lopsided decision.