Tuitt was great in 2012, a very athletic 2 gap DE. In 2013 he was too heavy and not as good. If he goes back to his 2012 weight then the Steelers have 2 very athletic DEs.
Tuitt was good in the NC game vs Bama. Nix was good in that game too. It wasn't the Notre Dame DL that was the problem in that game.
Here's a video of Nix vs Bama. You can watch Tuitt on each play. Tuitt is #7 and they switched him between LE and RE, so look for the single digit guy next to NIx on each play and that's Tuitt.
On a side note, people wondered why Barrett Jones fell in the draft. Nix destroyed him.