I knew this discussion was coming and it's really way too early
First let me sing the praises of Serg. No one was more thrilled to see this guy leaving the Montreal Canadians organization than me. He has all the makings of an elite offensive #1 defenceman. Seeing him thrive with a great team in Tampa is such a better site than with that awful red, white and blue team to the north. Doesn't seem to be an overly physical defenceman but he has that 6'3 frame to grow into which will just give him such a better all around game the older he gets. Such a skilled, slick looking player.
BUT for overall value to a team I'm going with McAvoy. He's already clocking 20+ minutes a night for his team as a rookie, and to my eyes is already clearly the 2nd best defenceman on the team after Chara. He's so well rounded between his skating, passing, stick-work and a low-key love for the big hit that there isn't any serious weakness to his game. You can throw in his stellar WJC play and tenancy for clutch play (for what that's worth) that he has me super excited for the future of our blue-line.
So from what I've seen so far McAvoy is already proving to have the ability to log big minutes and play against tough competition. Not that Serg doesn't have that same potential to do in Tampa, but he's being asked to play a much smaller role so far with the likes of Hedman, Stralman, Dotchin and even Coburn able to log more minutes than him. And that's probably just fine with Tampa, he doesn't need to be rushed into a bigger role than necessary so early into his career.
So overall both are going to be reeeaaalllly, realllllly good but throw my homer vote to McAvoy