I personally don't know of many businesses that exclusively book on one or the other.

I do know that almost every time I try to book on Westjet, Air Canada has a more convenient flight time available at no, or only slightly extra cost. For instance, while its nice that Westjet now flies YQR-YYZ, the flight leaves to Toronto too late in the morning, and arrives back too early to be of much use to a business traveller. Westjet will pick up all the grannies that want to visit the kids in TO, but I doubt they'll be picking up much high-yield, short-notice traffic on that route.