Lvl 6 artifacts offer another  10% increase to base stats like all of its previous iterations, and higher content of faction wars give better crafting materials for gear

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.

I think theres a lot more to be said on the subject.

For example - on gloves, chest and boots, it is possible to get HP/ATT/DEF stats either with or without %. 

It is much more desirable to have HP/ATT/DEF with % on the primary stat, than to have an item without %. 

Obviously this does not apply to items with Accuracy/resist/speed as the primary stat, as it is not possible to get % for these stats.

Also, it is not possible to get % stats on weapon, helm, shield - so these are less of a worry.

Likewise, % stats are preferable your secondary bonuses also.

As such, generally speaking 6 star items are going to be the best. Generally speaking, rank is also more important than rarity. 

Basically, a 6 star legendary item is going to be the best you can aim for - BUT if the primary and secondary stats are bad, then it is basically a worthless item, despite being top tier.

Hope this makes sense!

It is also worth bearing in mind that a 5 star legendary item with great stats can be better than a 6 star item with a lesser rarity or generally bad stats - but this is likely to be a close contest.