Karlsson plays a much tougher position defensively
Uh, centre is much harder.
You have to worry about your own lane, then young wingers. With Matthews, he's got Hyman that is a good worker and Nylander who is a Sithis-like void defensively. And he still produced 40 goals and could well top that this season.
I took Matthews. He has well above-average defensive game and a stellar offensive game. Just incredible player.
Karlsson is also elite at his position, but is too lackadaisical at times in his away-from-the-puck play, and that just should not be at this point in his career. If Erik were pristine defensively, it would, indeed, not be close. But the three zone game edges this in Matthews' favour, as well as his being a centre.