pat,
I generally base my flight decisions on timing and costs, rather than which airline to use. I've flown with both airlines out of Vancouver, although lately I try to use Calgary as much as possible as it's a bit shorter in my circumstances.
I haven't noticed too much of a difference between economy seats in Air Canada vs. Air Transat. They ALL feel cramped! I try to get up and walk around at periodic intervals during the flight to help alleviate the discomfort of the "flying bus". As I recall, Air Transat offers a type of "premium" service which is not first class but provides slightly better seating and a few other perks. I used the same type of seating on Zoom before they went bankrupt, and these were a bit more comfortable.
I used Air Canada on my flight to Paris last May, and I'm booked with them again into Rome this year. They usually seem to have the timing that fits my schedule, and their code share connections are quite useful. On the trip home last year, I found that I was assigned to Lufthansa flights out of MXP, rather than Air Canada.
One thing I've noticed is that the tickets are considerably more expensive this year. I waited for awhile hoping there would be a "sale", but unfortunately no such luck so I had to pay.
Virgin Atlantic is reportedly looking at routes into Canada, and WestJet is starting code share with several other airlines, so hopefully we'll have more choices in future.
Good luck and happy travels!