My current "which card" problem arises when buying take out food.   I get 5% (= 6.25% with bonus) using US Bank Cash Plus if the place is coded as a restaurant, but only 1% (= 1.25%) if it is coded as a  fast-food place, in which case it is better to use a 2% Fidelity Amex (or the 1.5% Cap One if they don't take Amex).  If I know in advance which place I am going to, I can look it up on the Visa site, but that is rarely the case (and is far too loser-nerd anyway!).

Eventually I remember that even if it say $30, the difference between 1.25% and 2% is about 23 cents, so this is not the most important financial decision I have to make!