For me it would be the level of Medi. That's the only reason I'd use Insight, it's a pretty average merc weapon otherwise.
My first Insight was in a normal non-eth Scythe with 33-61 weapon damage. The thing that made it worthwhile was the level 17 Medi and the merc's survivability in OK infested areas (It took at least 3 hits while IM'd to kill himself). My new one is an eth CV, with 17 Medi again.
The best Chaos/WSK Insight would have to be a non-eth Voulge with the lowest CS, AR% and ED% rolls, and lvl 17 Medi on a Prayer merc.