The answer will be different for different people, because it depends on what the traveller finds important. In your case, you might not yet know what you find important!

First, there are a few things to consider independent of choice of frequent flyer program:

1. Route network (where): Air Canada is a larger airline with more destinations, but it might be that Westjet flies where you want anyway. Consider also parter airlines. Air Canada also has a wider network of partner airlines, but Westjet and its partners may also get you wherever you need to go anyway.

2. Schedule (when): Frequent business travellers might value more flights per day on a route for flexibility and backup when things go wrong, but some people have fixed plans and don't really care. You'd have to look at the airline schedules to decide which one meets your needs.

3. Cost (how much): Highly flight-dependent. I've never found much consistent difference between the two airlines whenever I check on a particular trip, but YMMV.

4. Quality of airline: This is a purely a matter of taste. In my opinion neither airline is so obviously better than the other that it would make a difference in your decision. They are both fine in the North American context.

Then you get into the frequent-flyer specific issues, which include:

1. Earning and redeeming for reward flights on the airline and its partners. I would also include the ability to earn via credit card sign-up bonuses and spend. I'm not a great expert in these areas as I've never been interested in award flights, so I'll leave it to others, but I'm guessing that due to AC's size and partner network with J and some F cabins that it would be possible to do better with AC. But WS may have some redemption sweet spots I just don't know about.

2. Earning FF top tier status. Now more difficult to accomplish on lowest AC fares, so be warned that earning status can substantially increase your annual spend. I don't know how it works on WS.

2. Priority airport services (check-in, security, lounge). I really valued these services when I had the status. Some people don't care. I don't know what WS offers in this area. Many of these services can be purchased via memberships or premium credit cards for probably much less than it would be to go out of your way to gain the FF status.

3. Upgrades. AC probably wins in this area as WS doesn't really have a business class cabin.