I always nair when I go for down throw out of grab at 0%. Then after ~35% I'll start to up air simply because they start to bounce higher. Although, there are floaty characters that bounce higher - Jiggs for example - at 0% so it's best to always try an up air on them. That's one situation where it's better to up air than nair. But, that's not what you asked. So, let me answer your question about which is better. Neither. They're both very good and situational.
We all know that if you short hop into nair 1 then it'll autocancel. Then, you could jab into grab which is what most of us would do. You could also try short hop into falling up air which could convert into a grab. However, after a certain percentage opponents will start be hit too high to try to jab into grab. So, my point is that nair 1 into autocancel will always allow you to jab 1 into grab. No matter the percent. But, once again, floaty characters like Jiggs might bounce too high for this to work consistently. Hope this answered your question.