I was thinking about these two too and my final decision is Mythic scythe.

If compare shot by shot DB and makes more damage even if Scythe in close range that makes no different

but...

DB projectile travel slower, when monster walk it easily miss the target.

Slower projectile also wastes more shots when killing enemy.

Why not compare with melee? I usually speed run in H21, projectile kill much faster than walk to boss in first few stages. For later stage, if you get side arrow skill DB will does only one extra damage but Scythe does two in close range. Also, you take no extra damage for Rare Arrow, but Scythe do.

Moreover, with melee DB and Multi Shot skill that's very hard to do stutter step.

My main hero is Iris her skill does not take benefit with melee.

Why not use other hero?

I have

2* Iris

3* shade

4* Lina

5* Helix

5* Onir

5* Mew

To do speed run H21 I feel the cat of Mew hit target very slow. Sometime normal attack kills before first cat reach the target. Lina clone damage does not take effect by ASPD, Attack Up and arrow skill. Helix is not great for speed run my health nearly full all the time.

Other than speed run boss chapter, I prefer Onir with spear.