I'm in the same situation, Heroic Elegon MH, Amber OH--using 7.5/6.5. I've crawled through my logs between using Scythe/Scythe and HElgion/Scythe, and there's really not an appreciable difference between lost attacks.
Honestly, you'll probably be more affected by poor positioning when a boss happens to rotate as you start an attack on the boss and eat a Parry from the frontal zone.