Oh God, you really wanna get into advanced stats? You'd be hard-pressed to find any star player with bad possession stats, otherwise they wouldn't be a star... Especially not one with such a high OZ start rating...
But since you wanted to open the door for advanced stats, here we go. These are from 08/09. Admittedly I didn't do 07/08 because of time. They may very well favor Ovechkin, I don't know.
Corsi ES%/Fenwick ES%
Datsyuk - 60.4/59.1
Ovechkin - 57.3/56.4
Worth noting, both stats appear to heavily reflect the top teams as much if not moreso than the top players. Detroit and Washington make up damn near half of the top 50 players in Corsi/Fenwick. Mike Green's numbers are actually better than Ovechkin's. Green is not exactly heralded by most for his defensive play, and most would agree he is better defensively now than he was then.
Takeaways/Giveaways
Datsyuk - 89/49
Ovechkin - 60/107
Now, here's where it gets fun. We're going into this with the assumption that Ovechkin was the hands down better offensive player that season.
Offensive/Defensive Zone Starts
Datsyuk - 51.4%/48.6%
Ovechkin - 59.3%/40.7%
Even Strength TOI/Points
Datsyuk - 1152 minutes/59 points
Ovechkin - 1316 minutes/63 points
Power Play TOI/Points
Datsyuk - 274 minutes/36 points
Ovechkin - 428 minutes/46 points
Didn't bother with PKTOI as they had a combined 3 points. Datsyuk played a little under double the minutes. Now, if you'll allow a bit of speculation, apply Datsyuk's pace to Ovechkin's minutes and he hits 67 ES points and 56 PP points, putting him 14 points over Ovechkin...
So essentially, Datsyuk is obviously the better defensive player. And offensively when you consider the amount of ice time each was allotted, you can reasonably suggest Datsyuk was more effective offensively given the time allotted... All while having a much less favorable zone starts...