You need to check out a spend-reward calculator for Canada.
The MBNA flat 2% card is good if you spend in a lot of categories.
The PCF one you have is flat awesome as it's free and 3%. In Ontario at least, no, you don't get that 7c/l at Loblaw gas stations, just the regular 3% as PC Points. Yes you can buy gift cards with the points or with the card. However, there are usually better deals if you can wait for them (eg, we regularly buy Canadian Tire gift cards with a 10% discount, you just need to watch the flyers for Rexall or Sobeys or...)
If you shop places that take Amex, the Scotia Gold Amex is awesome (though it's travel). Otherwise, the Momentum Infinite VISA is good (4% cash back at gas/groceries, etc).
In terms of forex, Chase Amazon, Chase Marriott, and Rogers are the only 0% ones. Tangerine is only 1%, though - most of the others are 2.5%.
The Tangerine card is great, if you mostly shop in 2-3 different merchant codes (again, buying gift cards means you save).
The MBNA SmartCash is a bad fit because it is bonus only to the first $400 a month, and even the 1% is capped. I used to have one; it's ok but for a family, no.
I had the Marriott card but unless you WILL stay at a Marriott the Amazon one is better (2% on Amazon.ca!)