Bryna
First you assert the reason Transat is cheaper is maintenance. So not fuel hedging, or aircraft commonality, or pilot/staff salaries, or anything like that.
Then you wonder why Transat doesn't use 320s instead.
In fact Transat have 330s (the evolved 300/310) but have acquired more 310s because they suit their business model better, which means they can make money and offer cheaper fares. 310s are in service with the CF and are still being ordered.
Let's hope you never have to depend on Transat to get a family member home in a hurry when Air Canada (the original ticketed airline) would not get it done. I have had that experience and am grateful they were flying (a 310) that day.