Advice for a Mage

[Squidivan]

What arcane artifact is better in-general (events, farming, bosses, mobs), is it the 71 or 76 one?

[QuaseT]

What arcane artifact is better in-general (events, farming, bosses, mobs), is it the 71 or 76 one?

In general valley arti level 76 is better. The more mobs a map has the more dmg does the 71 infested arti add. So the longer the map, the more useful is the 71 arti. Personally, I would stick to level 76 arti for all maps if I had to decide for one.
For the temple event and vardan gates elite its useful to have both if you plan to go for lb. For everything else just 76 arti is more than enough.