Malkin and Crosby don't play together. Malkin is like Dagestan and Crosby is like Chechnya. In the same neighborhood, say hit to each-other, share similarities but are separated.
Crosby is king. 21 points in 12 games last year, McDavid 9 in 3 seasons. No contest.
It's better if you don't play together because it causes match up nightmares for other teams, they can't just zero in and game plan for one line.
Crosby in his first playoffs had 5 points in 5 games, but was shut out in the pivotal game 4 and 5 as the Pens were ousted by the Senators. That was about a 0.5 ppg drop off from his regular season PPG.
McDavid's drop off was virtually identical.
Gretzky also had a fair sized drop off from his regular season stats to his first playoffs.
Playoffs take some time to adjust to even for great players. When McDavid is 27/28/29, I would naturally expect higher playoff numbers for him because he'll be a much more mature player by that time.
No one "masters" playoff hockey by age 20.