It's tough to say who had a better rookie season between the two players. Personally, I think it's a toss up.
100 points in 73 games
Registered a point on ~36% of his team's total goals
102 points in 81 games
Registered a point on ~42% of his team's total goals
-Mario played 8 fewer games than Sid, which significantly decreased his percentage of points to total team goals.
-There was more scoring in the NHL during Mario's rookie season than Sid's. It was more of a run-and-run style back then, as opposed to the various types of defensive systems that are employed these days.
-Mario played his rookie season with the 2-line pass rule enforced. He surely would have piled up more points without it due to his knack for getting breakaways.
-Sid faced WAY more pressure than Mario did during his first year. Sure, Mario was in the spotlight, but it wasn't even close to what Sid went through.
-Sid was surrounded by better players than Mario during their respective rookie years. I know that most of the veteran signings from this past year were a bit disappointing, but Mario truly had nothing to work with.
So, like I said, it's a toss up.