Tough call - as others have pointed out, both Vance and Rose are built to be MLB in this system, but it's also clear they are two of the best 3 LBs. McMillen and Hummel are probably better than either in space in the passing game on the outside, but vs. the run game and getting after the QB Vance and Rose are a big upgrade.
It's hard to say, since the defense hasn't really had many bad quarters. The question isn't whether Rose or Vance are better at MLB, it's what two guys do you have out there at MLB and SLB to make the D play best? The coaches seem to think Vance at MLB and Rose at SLB is best.
My guess is next year we'll see Vance at SLB and Rose at MLB. I think either could end up being good at the SLB if they were there all camp. Not sure how much time and how many reps Rose or Vance got at SLB, but it does seem like the staff made a bet on McDonald at that role that didn't quite play out as expected.