I don't think 'regret' would come into much of anything for decision-making when it comes to expansion. Montreal also has multiple people who have directly reached out to the NBA (as confirmed by Silver) and Molson also expressed interest in basketball in years past.
By the way, I'm not strictly talking about sports when it comes to both cities (and even so, uh, Montreal also holds stuff like the Grand Prix?) but just generally. Montreal is both a far bigger market in terms of people, the city itself has far more international renown than Vancouver when it comes to general culture (art, politics, multiculturalism, history, technology, etc.). It's just more massive in every way. To me, it's a no-brainer. I don't see a single advantage that Vancouver has over Montreal. What does Vancouver have that Montreal does not?