Diurnal Sect has numerically superior effect, but will result in more overheal than Nocturnal Sect. However, as era1Ne points out, the Diurnal HoTs are subject to extension by Dilation and Opposition, and that makes it the go-to sect when you want to spam Gravity for a whole forty seconds at a time.

If you know you can pull it off before the tank is in aggro range of the next encounter, you can switch to Nocturnal immediately after the last mob dies, shield the tank, and switch back to Diurnal. Probably no one will fault you for not switching back and forth for this, though.