the Nickel is really not that great and if you wanna know why go watch the 49ers vs cowboys divisional championship game. It leaves you extremely vurnerable to the run and passing is not that difficult either. I prefer using a 4-3 because i have a great d-line but i hate facing 3-4 sets. TBH the 3-4 is probably the best pro defense there is because having an extra linebacker is often more useful than an extra d tackle. It shutsdown the run and pass fairly well. If you get in a situation where you need a different defense go with the quarter.