The difference is Grealish is often crowded out and doubled up on, and the majority of his short passes are the probing, risky type that leads to a tap-in for Ollie. Those are in dangerous, congested areas and are more likely to be cut out, but also cause havoc for the opposition. Context is key here.
FWIW I like McGinn, he's been brilliant for us. Given his journey with us if he's already hit his ceiling then he's done fantastically well. He will always be remembered fondly of he was to leave.
I still think he can do a job, though. Whenever he's had time to rest up he's come back looking like a world beater at times; he would benefit from some healthy competition.