If you are in the air then I would say Nair for multiple reasons, but first I must address this:
Well if you're getting pillared by falco
Pillaring is a shield pressure technique, not a shine combo.
Okay, Nair is what I'd say then. Not so much it is a good choice, but that Up B is a worse one. Up B covers a mostly vertical ground with about a thirty degree angle. So if they are anything but above you, Up B wouldn't be a good choice. Nair lasts much longer and covers the front of you. What would I choice over both Nair and Up+B (if I was in the air)? Uair. Quicker, decent priority, covers a largerer arc, more range, quicker in general. Or airdodge.
Overall, given Up B and Nair, I'd say Nair unless you are Up Bing out of shield. Given Up B, Nair, and Uair. I'd say Uair.