Definitely one of the best.There is an argument to be made for weapons that have build in multi-attack functions such as repeating crossbow too. The main advantage of Aetherius comes from the nature of AOE hits like Mark of the Hero, because only the first hit will be aoe, the rest remain single target, so Aetherius will always do high aoe damage while repeaters can scale up to be comparable in single target damage.
But well, hardly anything survives 500-1500 hits, so it's mostly theoretical difference.