We flew Air Canada May '04
We flew WestJet Dec '05
The difference? Not much.
They both flew nonstop from Calgary to LAX. (2hours and 40 minute flight time)
They both gave free beverages and snacks (sesame snacks on AC, cookies on WJ)
Both sold sandwiches for around $5.00, and candy.
Both had good service, nice people.
Prices were the same. They may look different on the web, but they will match each other, so go with the one with the flight times that work best for you. (mind you, when we flew AC, we got Kids Fly Free, which was soooo awesome. WJ also puts on good seat sales, check it everyday)
The planes were the same size (3 seats per side)
The only difference:
Westjet had leather seats, and you get your own tv on the seat ahead of you. Bring a set of headphones, or you can buy some on the plane for $1, cheap ones, or $3, better quality ones. You get 24 channels to choose from. This was an awesome feature. The kids just watched tv almost the whole time.
Westjet also always tells you a joke. But AC does now too.
My brother will only fly WestJet, he believes they are the best.