I don't see the reason for the comparison because these two teams are operating in two different levels because United is currently undergoing a rebuilding and is just trying to get back to their feet united has changed different managers in a space of about three years that will help you to know the current state that Manchester United is united is not fighting for any title as far as am concerned
Arsenal has been under the control of one particular manager for the past four seasons so ordinarily they should be better than United because arsenal has a project that they are having and that is winning the premier League title so for the past three to four seasons arsenal has been better than United because United has been struggling and trying to finding their place in the premier League. But looking at it holistically I don't even see any difference as far as am concerned between the two teams because both arsenal and Manchester United has not been able to win the league in the past four years because both teams has not done well
Just like you said, Arsenal and Manchester United cannot be compared together because for more than 3 seasons now, Arsenal has really gain more quality structure than Manchester United. The only problem Arsenal is facing is lack of tactical stamina to withstand their strength for a long period of time and that has been the reason why they haven't been able to win the league title for quite some seasons.
Arsenal being under control by one manager for 4 seasons also shows that the team is in a considerate form unlike Manchester United that has been jumping from one coach to another yet no success or improvements. Even if Arsenal and Manchester United has not been able to win the Premier League for a long time now it still doesn't mean that both team operates in same level and if it is to be looked at through titles, Manchester United has got more Premier League titles than Arsenal but in recent years, Arsenal has performed more better than Manchester United.