The AC experience is based largely on two factors:
1 - The individual flight crew
2 - Where you fly out of
AC does have some great crews. I know some of them personally and many do their best to make the flight enjoyable for the passengers. However, AC morale is very poor. As evidenced by the recent strike vote, management and crew are at each other's throat and AC crew have often had to sleep in the terminals because of managements poor planning. Considering this, it's little wonder that you can get some very bad crews.
Westjet, on the other hand, are owners in their airline and that not-so-little fact is a major improvement on morale.
Connections for AC if you fly out of the East or Vancouver are great. If you fly out of the prairies .... not so much. They route most flights through Toronto or they use one of their Star Alliance partners and route through Denver, Minneappolis, etc ... in which case why book AC when you can book that US airline for far cheaper?
Westjet has great flights out of Calgary and Vancouver ... out of Edmonton, though, they're really lacking (I'm not sure how their flights out of the East are). We used to have great connections, now they've gone seasonal with those. I'm hoping that the new terminal expansion will bring better (and cheaper) Westjet flights.
We prefer to fly Westjet over AC. In roughly 30 flights on WJ, we've never had a bad flight crew with them (though I have heard of others who have) but with AC I find it's quite likely we'll have a bad crew (I'd estimate 2/3 of the 50+ flights I've had with them). That being said, we find ourselves flying the main US carriers more often because they have often been much cheaper even when we add in having to pay for baggage.