Not interested into "whose better" debate, but think of two things, where Mara seemed to have clear advantage to Messi regarding his time. Ball was heavier and easier to control at least so it seems in old videos. I've actually played with some heavy non-modern footballs and could manipulate it much better than modern bouncy-floaty ones. They also decelerate rapidly, which makes push-and-run more comfortable and passing easier to judge, first touch more soothing, able to stop the ball dead in an instant with heavier touches. Pitches were awful though.
Other thing is almost no collective pressing. In most videos he deals with players one at the time like fighters in old kung-fu movies, not having to go against tactically astute groups that close down passing and moving lanes like swarm of wasps. Think individually defenders seemed also much more passive and bad at positioning. They were reckless, yes, but that doesn't mean skill.
Not taking away anything from Diegos genius, just couple of observations that haven't seen mentioned, when these era comparisons come up.