I think it depends what char you're playing.
Pain Inverter is a more passive skill to use, fire-and-forget and casting it on recharge. It fits on a caster bar better than melee. The foe does the damage to himself.
For melee chars Asuran Scan works very nicely, since it increases the damage output quite heavily, especially with Asura maxed. It will also form part of your normal skill setups and activations while in battle while you are now putting out damage, especially with having Aura of Holy Might already. You can easy go into 100+ damage digits with these 2 skills. And with enemies in a ball around you when you hit, watch the pretty yellow numbers.
I like both.
Pain Inverter I normally use if no other skills are available from primary or sec lines, especially on casters.
Try both and see for yourself.