A 50 goal season in Bossy's day is like a 38 goal season in recent years. Just talking in raw numbers is misleading without the context of how many goals were scored per game in the different eras.
Bossy scored 60+ goals in 5 of his 10 seasons, plus he had 58 in 76 games one year and 51 in 67 another year(ie he probably would have cracked 60 if he played a full season both those years). So that is basically 7 out of his 10 years he wasn't just a "50 goal(that would equate to a 38 goals)" scorer
Bossy is the best goal scorer of the bunch, but I think Lafleur's total offensive game is still a little better.
Bossy played 10 seasons and had a PPG or more in every one. Not counting his last 19 games played year in Montreal or his return in the late 80s, Lafleur had 9 out of 13 PPG seasons.
Bossy had 7 115+ point seasons(averaging 124 points in 77 games), Lafleur 6(averaging 128 points in 77 games). (For the sake of fairness Lafleur had 2 injury prone seasons that prorate to 101 and 110 points, Bossy minor(7 games missed) injury season that prorates to 100). I really don't see how this makes Lafleur any better, especially if we start valuing goals more then assists. Only way Lafleur stands out here is if you look at the totals, Bossy had the best season(147), but Lafleur would have the 2nd(136), 3rd(132) and 4th(129) best seasons before Bossy's next best season(126)
If i had to make arguments for each. Bossy was way more consistent during his career(albiet shortened one) but especially early on. For the case of Lafleur I think Bossy benefited more with his linemate(Trottier, who was the best linemate either player had) so you could argue Lafleur generated all of his lines offense