Potvin played 6 seasons there, Joseph 4.
Potvin started out brilliantly as a rookie but his play steadily worsened from there out. Was really let down by a brutal glove hand where he'd stab at the puck it rather than just letting them hit it. By his last 2 seasons in Toronto, he was considered generally mediocre, and that's why he was replaced with Joseph who turned the franchise around immediately.
Given that his last couple years were very spotty, I don't think his longevity really gives him much of an advantage.
Joseph also took the Leafs within 2 games of the Finals so not much difference there.
As someone who absolutely hates the Leafs, Joseph was to me a far bigger asset to that franchise through his time there than Potvin was. Was much more critical to their playoff success than Potvin - Gilmour was the real engine of the Potvin-era Leaf teams, whereas Joseph really carried those teams deep into the playoffs on a couple occasions.
