My 2 cents
PetroCanada.
My family is PetroCanda customer since early ninety nineties.
Reason: Better, especially its additive (Subjective)
(Shell, Esso and others have their own additives).
If you have CIBC PetroCanada Credit Card, each litre is cheaper by 2 cents, and you also earn Petro Points.
If you are CAA member, then you have extra 20% Petro Points, and you can redeem your Petro Points towards your CAA membership fee. (Better visit Petro Canada site).
As for Costco, it is only 5 cents per litre cheaper? And if Costco is very close to your home, then, you can always fill up your tank when there is NO queue waiting. Imagine if you have to drive 10 km to Costco (round trip = 20 km), and because it is city driving, then you already spent 2 litres of gas! Also, if there is a long queue waiting, Idling your car is bad to your car (and WASTE gas by ?? litre).
Filling 40 litres of gas at Costco seems to save 2 dollars, but the cost of 2 litres driving there is more than 2 dollars, if comparing to PetroCanada, the saving is only 1.20 (and there is NO petro points, and I doubt Costco has any additive), and also you don't have to idle your car WASTING gas.
I usually redeem Petro Points for car washing (6700 points per wash, cost per wash is over 10 CAD).
My 2 cents.