Marcos is pretty competent in almost every area of the game, proper all rounder, maybe quite weak defensively but he's improved as the season has progressed. He's better in almost every area of the game than him though. I think the only real thing Marcos has over him is his height. Emerson is having a breakout season at Roma, proper complete full back. Doesn't have a real weakness. A lot of Roma's attacks go through him on the left, but he's also quite strong defensively.
If we compare him and Alonso statistically he blows him out of the water:
Some maps comparing him to some PL LBs/LWB.
He'd be a very smart, Azpilicueta-like signing I think. Still hasn't his prime yet either so is also going to get better.
Edit:
@manpe I forgot to actually add the images. Also added images of left backs with similar profiles to him.