Artifact Star Values vs Rarity

In my limited experience, also according my research and most experienced players in my Raid discord:

Stars have more value than rarity, even when it comes to long-term leveling up of the artifact.

That is certainly what my own experience confirms so far: I have levelled up both a four star rare and a three star epic of the same artifact to 8 several times, and the four star rare remains the better of the two.

This is supported by the price of items in Market, and the cost to unequip artifacts. Note that a common item with four stars costs much more, in both cases, than a one or two star Epic. This implies to me that the game makers specifically intend stars to be more valuable than rarity.

A quick additional note about artifacts:

Percentage stat boosts are better than flat stat boosts. So a 150 HP boost is inferior to a 5% HP boost even if the latter adds up to less than 150 HP on a specific champ.