The answer to your question isn't so simple.
The general rule is a 6 star Artifact gear is better vs. 5 star Artifact Gears.
However, this general rule doesn't always hold true in every situation.
5 star gear can be better vs. 6 star gear due to having super lucky rolls.
It all depends on the situation.
For Example:
Lets say you have an Apothecary.
Most people want their Apothecary to be insanely fast.
The only thing which matters is Speed.
Now, lets say you own a 5 Star Speed gear with Triple Speed Roll
A 6 Star Speed gear with Triple Speed Roll should hands down beat the 5 Star one.
However, what if you are super unlucky?
What if you can't get a 6 Star Speed gear with Triple Speed Roll?
Than the 5 Star gear will be better option.
It will be the better option because it is giving your Apothecary the most speed.
Will the gear always remain the best?
Well, Probably Not!
1 day - you may get a 6 Star Speed Gear with Triple Speed roll.
Than your 5 Star gear will no longer reign supreme.
But it is unclear when this will happen.
Your 5 Star gear may reign sumpreme for weeks or months, until you acquire the 6 Star gear piece.
This is the "Grey Area of Artifacts or Accessories"
You definitly have to evaluate the situtaion on a case to case bases.