You do realize that humans have physical primes, right? As do NHL players.
I think anyone who can use some logic would say the 22 year old who outproduced the 27 year old likely has a better shot of improving and becoming a better NHLer.
Both the players you listed are a year older than McMann... I like McMann, but he is approaching 30.
Also, Mason Marchment is the most overrated player on these boards, he is not that good.
I think McMann is better (even if it isn't by a lot) and he is much better in the role he plays, so I wouldn't advocate for Robertson over him.
The players also need time to develop in the NHL, if they aren't your best option come playoff time, fine, but how will you know unless they get playing.