[Eleven's] doing exactly what Mr Copper said, and deciding who wins and who loses. Is this meant to show us that Eleven is more well-adjusted than Ten, who had the same power but let it drive him over the edge?

Now that's an interesting question. Nine rejected it outright. Ten fought that power all his incarnation too, until he hit that final no-win in WoM. He destroyed the Time Lords--twice--because they'd embraced that power wholeheartedly, and then sacrificed himself for Wilf because he was starting to embrace it again as well. Now Eleven has embraced it, seemingly with no regrets. Maybe that means he's come to terms with who he is post-Time War, or maybe there's something else, hinted at in "Amy's Choice," which hopefully will be explored. *crosses fingers*