Exu S3 vs Ash S2 is very situational but the TL;DR is Ash is better at bursting a single enemy (or several who are close enough to all get hit by her flashbang) and better at dealing with armor from 600 ish upward. Exu better at dealing with multiple enemies who come in waves or are not stun-able. Ash is also more likely to have her skill up at the right time due to smaller burst window and manual activation.

When factoring in buffs this changes slightly because now Exu can actually deal with some armor (scale up the armor cut off as needed). But for the most part the same reasoning still apply because they both scale extremely well with buff.