Companies only pay taxes when they make a profit. That profit gets shared with the government and shareholders (private or public). So the rate of taxation is quite irrelevant to the company's bottom line as it has expensed as much as it wants before having to pay taxes on the rest. AC's fares would not drop were it to locate in any other province for that reason alone. AC's management could decide it will lose money every year and cut fares accordingly. It wouldn't be paying taxes, or dividends, so the only consideration becomes what it wishes to add to its cash reserves. As for unions, AC's are nation-wide and with more unionized employees located outside Quebec than inside, that situation would be same wherever AC was HQ'd.

There were two reasons many companies moved their HQs from Montreal to Toronto, neither had to do with taxes or even unions. One was the evolution of Canadian business towards Toronto as a corporate centre and increasingly a manufacturing and service centre for the country. The second was the perceived political uncertainty in the 1960/70s around separatism which led to the banks to move since they were uncertain how an independent Quebec would affect their standing as Canadian chartered banks. (It was easy for a bank to move since there's not much gold kept in the vaults these days.)