Skelly is way more talented than AMN. Much better technician with better vision and decision making. AMN is probably a better athlete, but not by much.AMN's problem is that he was never a 90 minute player in terms of focus and concentration. I too felt he should have got more midfield time, but I can see why coaches might not have been keen, as he had a habit of out of nowhere playing loose passes in central areas. AMN has always been a bit of a street footballer - amazing physical and technical gifts but not the mentality for the very top level. Personally I would have tried harder to keep him as a swiss army knife, but he was caught between not having quite the technical level to be a winger at the elite level, not quite the vision to be an attacking midfielder, not quite the focus to be a DM and not quite the defensive chops positionally (despite being excellent 1 on 1) to be a full back at the elite level.
I reckon in the 80s, 90s he would have absolutely killed it in central midfield, but the tactical side of his game wasn't quite strong enough for the current elite level.
MLS seems less loose in his style and naturally more intense/driven. Might not be as talented, but looks to be the kind of personality Arteta loves.