It's not whether you fast fall. If your opponent doesn't get hit, they have the time get in position for an edgeguard of any sort. Using a bair, you can be down and up without much delay. Not to mention, bair is more powerful and has better range, and you can actually fast fall it, making it overall faster. Nair comes out slower, can't be fast falled, has more wind down (thus is more punishable), doesn't have as good range, and is less powerful. It is in everyway not as good as bair in this situation. Remind me again why you would use it?