Either striker or attacking midfielder, for different reasons (so I'm not helping you to settle the debate; I apologize)
A striker is the player highest up the pitch, which means that he plays in the least amount of space, as defined by the offside line. He plays primarily with his back towards goal, which is the most difficult way to play. When he gets the ball facing forwards, he is generally expected to do something conducive to attacking play. He must have great offside awareness as the line is very quick in the modern game (this is a major reason why it will not do to simply stick a quick winger up top, a la Di Maria, though van Gaal had a good reason for doing so). He must also be able to hold the ball in one way or another (through physicality or dribbling), as well as release, and have a good understanding of what sort of balls to expect from his teammates. Finally, he must be able to finish, which takes a level of composure that most players will never have.
An attacking midfielder, on the other hand, must be able to make precise movements off of the ball, whether that means moving to where the play will be (into space, presumably) or being agile/quick enough to lose one's marker in a controlled way. He plays in the hole and will often receive the ball while facing forwards and under a great deal of pressure, so must take his first touch into space and be able to maneuver (dribble) away from the defenders. He must know where his teammates are at all times so that he can receive and release the ball in as few motions as possible. He must understand space, and the ways that movements can open it up. He must be able to identify when a defense splitting pass (one that cuts out defenders/passes through lines) can be played, which in my opinion is the most difficult single skill to consistently display. On top of all of this, he is generally expected to be able to shoot from range (to force the opponent to pressure, opening space up for the forwards).
Personally, I believe that, while attacking midfielder is generally a more difficult role, the best of strikers display a skillset that is rarer than that of the best attacking midfielders. I think it is more difficult to be a truly world class striker than a world class attacking midfielder, but easier to be a good striker than a good attacking midfielder.
To add on, Messi possesses all of the above attributes at the highest level, which is why I believe him to be the best.