From 1971 through 1975, the Bruins were considered better. But as has been mentioned before, they only met once in 1971 - which ironically was the best Bruins team in history.
From 1976 through 1979 I think the Habs were considered better - including those ridiculous Hab teams of 75-76 and 76-77 where they only lost 11 and 8 games, respectively. But the two teams were consistently atop the NHL standings for pretty much the entire decade of the 70's. I have always said that but for the Habs being such a dynasty in the late 70's, the Bruins would have won maybe 2 more cups that decade....Don Cherry would have a couple of cups to his name.
And don't forget that Orr was basically done by 1975 at the age of 27. He stays even remotely healthy and those Bruin teams of the late 70's would be that much better. Trust me, the 70's were a really frustrating decade to be a Bruins fan, even though they went to the finals 5 times (winning twice) and were very, very good. There was always just something in the way to getting over the hump.