So overall I would have to say Stosur though Petrova does come close. The main difference would be the fact that Stosur has proven to have better nerves under pressure and thus would more than likely win more often than lose this matchup in my opinion.
I don't think so. Actually Stosur has much worse nerves, you just can't judge it by one single match that happened to be a slam final.
But I believe it is Petrova becoming more mature than actually becoming more psychologically stable (not that both things are much far away from each other). At her peak Petrova won 3 consecutive titles and 6 consecutive finals while I doubt Sam has ever won a match in the next tournament after a breakthrough. You can even count that Petrova has beaten Wozniacki in a final in a 6-2 6-1 manner, this is quite hard to happen at any level.
Backhand goes to Petrova by a large margin. I mean - everyone has a good shot at their peak. You can even look at Pironkova, who has an abysmal forehand for a top 50 player, but still I remember her stringing 3 or 4 consecutive winners from her forehand in her match against Zvonareva last Wimbledon.
I suppose Petrova can eat Sam's serve as it is great by speed, but still somehow one-dimensional. In addition, Petrova is nearly 1.80m, so it won't get that much of a damage for her.
And last, I believe the margin between their stamina is much bigger. Petrova gets much more easier tired if a match comes to a 3rd set, but a usual Sam is more likely to choke it than to lose it because of fatigue.