Why not use archer for long range take downs and use daggers to kill people silently close quarters. That's what I do. I prefer the silent strategy myself..
As an illusionist, you kind of have to limit your leveling, or your spells no longer work. As a result, I'm trying not to willy nilly improve every skill. Besides, bow do a lot more damage than daggers, even if the daggers get a much higher multiplier.
As it goes, I do use sneak attacks most the time and most the time they are one shot kills, and when they aren't, I can pacify and do it again, or go invisible and do it again.