I measure the quality of a manager in the macro, not the micro, meaning how much of an overall impact they had on the sport.
When talking about greatness, lasting influence should matter, particularly in an occupation such as coaching, where winning trophies can have more to do about the financial backing a manager receives versus their actual ability.
This leads me to Wenger > Ferguson.
Go ahead, tell me what lasting impact Ferguson has had on the sport . . . I'll wait. Are there teams mimicking his tactical setups or playing styles? Does he have a stream of former players and assistants coaching at high levels? Go ahead; describe one thing Ferguson's teams were known for on or off the pitch that teams still utilize today.
Ferguson excelled in an era where coaching was about motivation and man-management, not tactics. It is why there is no lasting influence his teams have had on the sport despite all the trophies they won. You could even argue that Ferguson's retirement came right as the sport evolved into the uber-tactical modern age, an area where he had a clear weakness. Evidence of this is the two Champions League final losses he suffered to a young Pep Guardiola in 2009 and 2011. Wayne Rooney went on to say he felt as if Ferguson committed tactical suicide as all he told the team was to press high and attack. The same generic tactical advice a 15-year-old with experience playing FIFA would give you.