I think Em would win... let me explain:

Nas is a dope emcee, one of my favs, but on a straight out battle Em would come with humor and hard punches. Nas would probably take the attitude that Em is just a little white kid who isn't from the ghetto, doesn't know about the rap life etc. That is fine and it might be true, but you can't win a battle by doing that.

With all that being said, I actually like Nas better than Em and I think if they both had to write a song, not a battle, on the same topic Nas would do a much better job than Em.