Its a bit hard. I'd honestly lean more towards Kurenai but Ibiki definitely has his uses. She's cheaper for one. Her mirror and 45% chaos is pretty good as well as when you get her standard to put targets to sleep.

Ibiki is good because of his unclearable immobile and his debuff of resistance/defense and more damage to immobile/torture units.

I would honestly say, why not both? But, Kurenai is definitely in more events and is the cheaper option.