The penis measuring contest is not doing either team any good.
Watching both teams this season, Arsenal are doing well. Yes there are still questions about whether they can last the season. It's normal for questions to be raised after playing poorly two games in a row (Southampton and PSV). And someone mentioned it, but it was also good to have Forest over the weekend to bounce back (The team looked like they barely got out of second gear for that game).
United have also done well and bounced back since the terrible start. In some games, coming to the CAF you can also see fans getting frustrated at the team's play some times. and that some games, the team team just about made it to win or played poorly (Southampton, Newcastle, Sociedad - to be fair, Arsenal drew at Southampton and have yet to play Newcastle).
I'd argue it's reductive to say who's going to finish above who at the end of the season, because it takes the fun out of the measuring contest highlighted. But for now, as they say, it's better to have points on the board then the hypothetical points that the teams will "definitely win" because of so called easier fixtures. If anything, over 2 decades of watching soccer is that odds and probabilities go out the longer the season goes on. So just enjoy the ride (because it's been a while since both of these teams played good proper football) and then let the bragging rights speak for themselves at the end of the season.