if you want to just get the rating and go, ATP will do just fine- I did a couple ratings with them and as long as you are prepared to self study(which is A LOT!!!!!) remember the guy that is helping you with your instructor rating maybe just got his the week before and may or may not be as knowledgeable as you. So you might find the knowledge portion a bit lacking. If you want to be a good Instructor I would find a school with an instructor that has been instructing for 3+ years, someone who can teach you how to teach and most importantly someone who has signed off CFI before and his pass/fail record