It won't matter much since a cup here and there will have negligible effects. A typical cup has at most 100-200mg of EGCG so you'll need to drink 5-10 cups per day to get an effective dose. You are still getting beneficial compounds nonetheless.
You also need to watch what you consume with it if you are looking to get more than a tasty drink out of it - things like milk/casein reduce the availability of some of the beneficial compounds.
So if you are looking to maximize the benefits by getting the most potent amount of EGCG, polyphenols, catechins & antioxidants, then purchase an extract to use in addition to.
It is the mark of an educated mind to be able to entertain a thought without accepting it.