Flown both, like both.
West Jet - flown them on 4 legs (ie. 2 return trips) and only had a tv on the backseat once so don't make that your deciding factor on flying them. They also are the only airline that I've ever travelled that actually weighed my carry-on and told me to check it or lose some of the stuff inside it (it had some books in it...) so watch your carry-on weight. Really don't like the fact that you can't assign seats at the time of booking.
Air Canada - I like 'em. They assign the seats ahead of time and they've got lots of flexibility in scheduling time. Their cancellation/change policy sucks though. West Jet is way ahead in that department. Like West Jet, I've had one flight with a tv on the backseat.
My decision is usually based on two criteria....price being first and then seeing if their flight times fit my schedule. I'm flying to Las Vegas in December. I ended up on Air Canada because their schedule was better than West Jet's, the price (after some patience and perserverence) was better than West Jet's AND I got seat assignments for 4 people...something I couldn't do on West Jet. Now...had West Jet's prices been crazy cheap, I might have gone with them and sacrificed my schedule. LOL