Just got my AST up to level 63. I've tried both diurnal and nocturnal sects to heal with. Personally I prefer to play diurnal and having hot's like the white mage has. I'm better at healing stuff then having to predict what the future might bring when it comes to shielding people.

Both sects are completely viable. The sect you'll be using is probably the play style which suits you the best. Hot's vs shields basically

If you're in an 8 man party with 2 healers, then it depends on the other healer which sect you should be using. If the other healer is a Scholar, then go diurnal. Shields don't stack. With a White Mage both sects work. If the other healer is a fellow Astrologian, you both should be using different sects.