As others have said, Marquis is the better player. Barnes didn't do crap on Pierce last year, and while I love our captain, it is possible to make him have some bad games now. Barnes didn't do that.
Plus, the one really good statistical year that Barnes had he played with Nash, which inflates his stats. Otherwise, not only is he a mediocre player, he's had some bad 3-pt shooting years as well (below 30%). While Marquis likely couldn't do better, at least he doesn't jack them.
I'm also eager to see what Marquis can do with Nate. Marquis can be a little passive offensively sometimes; however, the steady poise he has could really compliment Nate's assertiveness.
Also, while I hope it doesn't come to this, Marquis could conceivably be our #3 wing. I hope we can do better, but if not, it's possible he can fill that role. He's been a starter in this league and scored nearly 14 ppg in a season.