I like to divide all exercises into two broad categories. THINKING and DOING. Basically we are either trying to isolate and squeeze a muscle or we are interested in blasting it with power and energy.
A pec deck is a great thinking exercise, getting a hard squeeze, getting your mind right into your pecs, feeeeling the pain. Bench press on the other hand is all about moving a big weight from A to B. We are not concerned with the feeling (apart from feeling magnificent!) of the chest.
So, before you pick up a weight to do a set know if you are thinking or doing.