For those who watched him... has he been more of an AM than a CM?
I watch Inter every week as my house-mate is a Inter fan. The answer to that question is dependent on who the other midfielders are. Hernanes can defend, but he is lazy. When Inter has tried to play a midfield of Hernanes and Kovacic with Medel they've been a bloody mess, likewise him and Guarin. Because they don't work hard enough in tracking back. However, with Marchisio and Pogba he should be fine as a CM. However, realise that he isn't a particularly good decision maker, and his passing is surprisingly sloppy for a guy with so much technical skill. When he has a bit of time on the ball he is capable of delivering fantastic passes and crosses. However, don't expect to see that regularly. Rather he will be most invisible and with the occasional moment of brilliance
As a treq/AM he isn't particularly good at finding space, as he isn't particularly good without the ball. He would rather run with the ball, or shoot than pass. I don't believe this is a great attribute for a AM. Given that Icardi tends to just sit on the shoulder of the last defender Hernanes tendency to be very individualistic tended not to bear fruit as there was no one opening up space, hopefully that will be different here.
However, Dybala and Morata are far more dynamic and unpredictable and this hopefully will lead him to be more effective in that position here. Just be prepared for him to ignore his strikers on a regular basis. This infuriated Icardi at Inter.
So the answer to your question is that he can play either, but he has strengths and weaknesses at both positions. The question is whether being surrounded by better players will make him look better than he did at Inter. Because if he performs like he did there people will hate him here. He statistically was quite impressive at Inter, but generally he would be terrible for the majority of the 90 minutes, and then produce moments of magic.
One last thing. A lot of people seem to be under the illusion that he is a creative player. He really isn't. He is a very direct player with a lot of technical ability, but he's no Pirlo.
edit - I see that you actually asked what position he played. A bit of both. Mancini even tried him as an inverted winger a few times (with terrible results). He played more as an AM though