This is a real nice matchup. Went with Robinson as I feel he impacted the game in more ways, but it's close for sure.
I have a ton of respect for Al MacInnis, who continued to add to his game as his career progressed beyond the big shot and enjoyed a very nice late career as the slower dead puck era suited him. I do think that his big numbers in Calgary were sort of a perfect storm situation with a very deep team and an all time powerplay, which MacInnis obviously played a huge role as an X factor (as a very recent analogy think of how the Oilers powerplay surged with Bouchard replacing Barrie because you must respect the shot and that opens up so much for the others).
As for Larry Robinson, I'm more familiar with his later career than the dynasty Habs days, but well, Robinson stands out for me for two reasons.
He is one of the very few seventies era players who aged gracefully into the late eighties/early nineties. You see so many of the others fall by the wayside with the massive changes in the league that took place throughout the eighties (compare to eighties players who en masse had extended careers through the dead puck era). Robinson may not have the wheels in his last years, but he had the smarts.
The other is just how dominant defensively he was. It was said about him in the late eighties "Robinson sees the ice defensively as well as any superstar forward sees it offensively" maybe a smidge too high praise given the Gretzky/Lemieux/Yzerman group back then amongst others, but drives the point home.
Robinson even during the dynasty days themselves was the one guy who could give the great Guy Lafleur a run for his money as the team's most valuable player. Obviously there is refinement in the later years as compared to the earlier, and Robinson's partner Serge Savard was generally considered the defensive mainstay on that pairing (though to be honest the little I've seen of Savard doesn't impress me to the degree his reputation). Scotty Bowman didn't like Robinson much himself, but then again, he didn't like a lot of great players, and Robinson seems to have reciprocated that feeling lol