Accurately measuring individual defensive impact from forwards isn't the most simple, since they don't actually have nearly as much impact as people seem to think, and their results are closely tied to the defensemen they play with, but Marner has a better 5v5 xGA/60 over the past 3 years (Marner = 2.36, Aho = 2.37) while he's been on the ice, and if we compare it to the xGA/60 of the defensemen they most play with, weighted to the amount of time they play together, to try to get closer to their individual impact, Aho just looks worse.
Now this method certainly isn't perfect, but even setting aside the overrated impact people think forwards have, I would ask, what evidence do you have that Aho is even better defensively?