I assume you mean when the nair cancels and then you do another aerial right away.
Up-air is probably better at lower damage where you know it will connect. It only comes out 1 frame slower than nair and you can do a lot more off of it. The main danger is if your opponent shields it, since doing the up-air right away will get you punished. Nair is a little more safe on their shield, but even then, you will probably get punished. I typically try to double jump if the first nair hits their shield.
I think this approach should be used very infrequently. If your opponent catches on, it is VERY easy to just jump up and hit you while you are slowly coming down with the nair (or with some characters, they can just up-smash you out of shield). It's best to use this when your opponent is shielding and under pressure from a bomb that has bounced off their shield. If you do it right, it will also shield poke. I still find it to be risky and something to only use once in a couple matches.