Hey Danny,
There are a couple aspects to your question, so I'll address each one separately.
1) Annual Spend
Before getting into anything else, the most obvious question is how much spend will you put on cards during a year? The answer to that has a big impact on what type of card you should be getting. Depending on the amount, it may not be worth it to pay an annual fee for the benefits you'd receive. As an arbitrary example, if the new-fee version of a given card earns you 2,500 points for $5,000 in spend, and the $120 fee version earns you 5,000 points for $5,000 in spend, you have to ask if the extra 2,500 points was worth the $120 it cost you to get them.
In order to do a full cost/benefit analysis for a card, you need to look at how it's benefits relate to your spend (i.e. if a card offers 5x points at grocery stores, and that is 90% of your spending, you need less spending to make the fee worth while). We can assist if you provide some details, but its also not hard to do yourself if you're prefer not to spread that information so publicly.
2) Variable value vs fixed value
Next up is how the various programs work. Some programs are have fixed values where each point is worth a set amount; in the case of you Scotia Amex, each point is worth $0.01. These programs are great if your goal is North American domestic travel where tickets are a few $100s, but they aren't useful for premium cabin travel as you'd have to save enormous amounts of points to pay for the whole ticket ($20,000 RTW ticket in first = 2,000,000 points). As the value is fixed, cost/benefit analysis is very easy. The annual fee on your Scotia card is $99, divide that by $0.01 = 9,900. So you need to earn at least 9,900 points per year just to break even on the card. If you don't earn that much, you're better having a no-fee card and putting the $99 towards a ticket.
In variable value programs, as the name implies, the value of the points differs depending on how they are used. For instance, in Aeroplan it takes 25,000 miles (+ about $150 in fees) to fly anywhere in North America. If you are able to redeem them on a route that is normally $1,000 round-trip, you would get $0.034 per point in value ([1,000 - 150] / 25,000), but if you redeem them on a route that is normally $250, you would only get $0.004 per point in value ([250 - 150] / 25,000). For business and first class rewards, the value per point that you can redeem for can easily approach $0.10 per point if you know what you are doing. Cost/benefit on these cards is harder as you have to take a guess at how much value you're going to get out of a redemption.
Given the differences, it's not hard to see why those of us that fly or spend a lot use variable value programs. However, to really get the huge value per point ratios, you need to be redeeming for international business and first class tickets (which start at 80,000 points and the really good ones can be closer to 200,000), so its not something that is possible for everyone.
3) Airmiles
I know one of the guys who was with Airmiles when it started, and his description was, "it's the best program in Canada, unless you want to fly somewhere". That doesn't mean people (tens of thousands of them) don't swear by it, but generally their program makes it very hard to accrue huge amounts of points and charges a lot of points for flights. Combined with some excessive redemption fees and the fact that you can only redeem for economy travel, and it becomes a program that is shunned by those 'in the know'. The fact that can offer BMO credit card holders a 25% discount on the required amount of points for a flight should give you an idea of just how unreasonably high their reward levels are to begin with.
4) Amex Canada
As you're likely aware, Amex is not accepted by many merchants in Canada. So if you choose to use an Amex as your primary card, be aware you're going to need a backup card. Personally, I carry an Amex Gold Reward as my standard card and have a no-fee RBC card as my backup.
5) General advice
Without information to suggest otherwise, as a student, I'd recommend you cut back to no-fee or low-fee cards. Your spend is likely to be low enough that the benefits you get from a high-fee card will be nearly completely negated by the fee you've paid. Save the cash, bank some points on a cheap card (CICB offers a $29 Aeroplan card) and keep your debt low. Do that and you'll position yourself to really get into the game after you graduate.
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