Westjet, can be cheaper, the same price, or more expensive, depending on factors such as fare & booking restrictions. As an example, currently for tomorrow on a day return YLW-YVR-YLW, the fare on AC is $327.42 - but the fare on WJ for the same day & route is actually $278.21 - cheaper yes, but not by that much. If you wish to book reasonably far ahead, and stay a Saturday night (in mid-April for example), the fare on AC is $138.03, while on Westjet it is $154.09.
Westjet has a great product, and is generally an excellent value. Remember though, that it seems every time AC lowers their fares to match a competitor they are accused of anti-competitive behaviour - that doesn't provide much incentive.