I dont necessarily disagree with you, as I agree that McDavid has more to grow into than Tavares did, in terms of upper end scoring potential.
That being said, I think you are wrong to say McDavid has better vision. Vision isnt just seeing passes, its also the ability to see open space when you are carrying the puck, and Tavares is absolutely elite when it comes to that.
When I looked at the pre-NHL John Tavares I saw three skills that had me convinced he would be a star:
1. Strength with the puck. Even when his skating was weaker, he was very strong on his skates.
2. Vision / Hockey IQ. His vision and hockey IQ (different concepts I know) were at the top of the NHL the second he skated on the ice for his first game.
3. Finishing ability. Outstanding wrist shot, excellent hand-eye, a menace in tight.
Its rare to find a player with vision and with finishing ability like Tavares, in fact I would say only Crosby combines those two skills better than JT.