Noct for Regular Pulls, Diurn for Bosses.

My reasoning behind this is that during any (most) regular pulls, the tank is the only target taking damage, Nocts potency boost is nice, I can normally ignore the tank after casting Aspected for the majority of the pull, and because the shield is an instant cast, it lets me react quickly to the tank taking spike damage (More time in Cleric), and an Aspected -> Essential -> Aspected is a great recovery combo.

Now, the reason I use Diurnal for bosses is because the universal pattern among bosses, is that the damage is widespread, however, in most cases, the AoE damage doesn't require a real heal, and the regen is more than enough for the DPS, so I always open with Helios -> Benefic on Tank, and go straight into Cleric and basically ignore everyone.

Ultimately, I love running like this because it effectively uses BOTH stances during a standard dungeon.