Butter and Butter Flavored Shortening
Butter and shortening behave differently in the cookie dough, so unless you are planning to do more adjustments, I recommend you stick to the given butter-to-shortening ratio. If a recipe uses both, it's often to get the best of two worlds.

This leaves the question of flavour.

If you use the butter-flavoured shortening, you will get a stronger "butter" flavour. This may or may not be what you want. So it's mostly a matter of taste (yours, to be exact) whether a trip to the store is in order or not. Without knowing more about your recipe, we can't even try an educated guess.