That's a tough one. Transat has a limited flight schedule (looks like 2 per week from Toronto. Not sure about Montreal) so your options due to delay/cancellation may be limited.
With that said, Transat's kids club (I'm assuming you're travelling with kids) gives you free seat selection, priority baggage handling, and early boarding.
Rouge is Rouge. I mean it's Air Canada on a budget. Small pitch, nickel and dime, etc. The flight attendants have nice hats though.
So if you're going in the winter, I think probably Rouge. Warmer weather, maybe roll the dice on Transat.