Uair is quicker, and will overall be better to use since at lower % you need to flow quickly between attacks/grab setup into attack. Many characters come close to the verge of "breaking" out with their own Nairs during these situations, and trying to land a reverse Bair will legitimately allow some characters or %'s to break out of hitstun and jump/counter with a choice.
If someone is in front of you, and you are trying to hit with the inside portion of Bair, so that they keep flying in front of you, you would probably have more success with the standard Uair or Knee. Weak Bair can punish poor DI better than Uair on occasion (since people tend to not fly as far from Weak Bairs), but if you have enough time to consistently accomplish weak reverse Bairs, you probably have enough time to regrab, Fair, stomp, or almost any other option that might work better in the end.
If you want an affinity for Bair, I'd probably suggest RAR or shield crossup attempts with the move, not trying to use it as a staple filler in combos