I just used WestJet for the first time. Here's an unscientific comparison with Air Canada from an infrequent flyer. Both flights were roundtrips from Toronto to Calgary within the last three weeks.
Air Canada assigns seats in advance. WestJet doesn't. You pick your seats online up to 24 hours in advance. This is more democratic but a drag if you're travelling without online access.
Winner: tie (each approach has merits and drawbacks)
WestJet boarded right on schedule. What's more, everyone boards at once. I thought this would create a bottleneck but this process worked extremely well: different parts of the plane filled at the same time.
Air Canada uses a class system and then fills the plane from the rear. If you're not Platinum Ultra-Elite Preferred and seated near the front of the plane, you wait a long time to board.
Winner: WestJet wins for being democratic
WestJet departed right on time. Air Canada departed 30 minutes late due to deicing, which is a good reason.
Air Canada is the clear winner with free headsets and movies. While going, ancient ceiling-mounted tube screens were used. While returning, passengers got their own widescreen displays.
WestJet has free commercial tv channels but charges $3 for headsets and $6 for movies. The low resolution screen uses the obsolete 4:3 format. On the return flight, I couldn't change channels from annoying Fox News (I like the one showing where we are on a map) and the screen beside me didn't work. There was no Off switch :(
WestJet had a restricted selection of meals for sale. Only nonvegetarian sandwiches --- luckily I brought my own. Air Canada had much more choice, including soup and pizza.
Winner: Air Canada
Flights were late for no apparent reason. The weather was fine (sunny and dry).
Tie: both losers
Attendants were pleasant and competent on both airlines. WestJet provided less service in French. Routine messages were prerecorded. The pilot's message was English only. Air Canada was bilingual, not that this mattered to me.
There's no clear winner. Each airline has strengths and weaknesses. Air Canada feels more professional and complete. WestJet feels more democratic and flexible.
For my next flight, I'll choose based on a combination of price and flight times. Maybe I should pick a different airline for each direction :)
Thanks to the competition, we all win.