I'm sure this is a tired topic, but everyone has an opinion on it, so I figure I'll share mine.

I'm by no means a frequent flyer, in fact I signed up for Aeroplan so I could get the $3 discount per flight leg by being able to decline the Aeroplan points. They'll probably expire before I get anything out of them.

Here's my take:

Neither has ever lost my luggage. Neither has given me poor service.

When I book, I generally go with whichever is cheaper. I suppose with Air Canada there's always the chance of getting bumped up to first/business class, but I would think with technology these days, they're pretty good at not overbooking (methinks), or at least good at not giving me any upgrades. Ever. Westjet, having just one class (Seinfeld's Elaine got what she wanted: a classless society), means no upgrades!

My Air Canada vs. Westjet Thoughts for Canadian Domestic Flights

• I do hear that Air Canada sometimes does overbook flights (the whole seat: GTE thing), but even when I have gotten that, I was still on my intended flight. Supposedly Westjet does not do this at all, so I guess that's a point for them.
• I vastly prefer Air Canada's seatback on-demand entertainment system to Westjet's 24 channels of TV. The internet has ruined my attention span for watching regular TV with annoying commercials and not being able to watch what I want on-demand. In fact, if both Air Canada and Westjet had the same fare, I'd go with Air Canada because I prefer their system.
• • Air Canada's has an AC plug and USB plug in almost every seat for easy power/charging. Westjet has neither.
• Air Canada is unionized, which isn't a slight toward unions, but there always seems to be a risk of one of their underlying unions (whether it be pilots, flight attendants, booking agents, etc), going on strike and messing up your travels. With Westjet, there is no such risk.
• Westjet doesn't charge extra for a sports bag, while Air Canada does, so that's something to consider if you're bringing skis/snowboards. Based on my previous experiences lugging skis across Canada, don't do it, just bring your boots that fit nicely, and rent skis/boards. If you can't tell, I'm not _that_ serious of a skier
• In my opinion, Air Canada has better food (though you do have to pay for it, I do like airline food)
• Air Canada's flight was once just slightly late, and they gave all of us a free beer
• Once I did have a Westjet flight get delayed on the tarmac due to a passenger count discrepancy (the count on the plane didn't match the flight paperwork, supposedly an errant ticket name change didn't take correctly, and that was a 1hr or so delay, meh, I think it was a freak accident thing rather than the sign of a genuine problem)
• Once I was on a Vancouver--->Toronto Air Canada flight that was empty, as in, no one in front, behind, or in the same row as you. It made for good nap time.

When I'm booking, those are the only factors I really consider. Simply because one person has a horror story of one or the other doesn't mean that particular airline has a higher chance of causing a horror story. I think the employees behind each genuinely do what they can to get you to your destination.

If I were going to the US, I wouldn't think about flying out of Canada, I'd drive to Buffalo/Niagara Falls (or whatever border airport equivalent you have, I feel bad for those from Edmonton/Calgary). Sometimes a domestic US flight is cheaper than the taxes on a US-->Canada flight. I wouldn't doubt for a second that Air Canada and perhaps Westjet are very top-heavy organizations, I simply don't know why popular domestic Canadian routes have such comparably higher fares than a similarly distanced US domestic flight.

Protips: The wine is often the cheapest alcohol as measured by alcohol content. Consider getting noise-insulating earbud headphones if you're a frequent flyer. They're what I use, and they function as excellent earphone/earplugs. There are cheaper ones than that one, but using them as much as I do, I paid for quality :)