Looking at the numbers, and using the experience I have playing AST, I'd say that for single target heals, using either sect would be fine, but Noct AST has a significantly harder time with any kind of AoE due to lacking a medica II equivalent and hence has a harder time in endgame content than Diurnal AST.

In addition to that, even though CU isn't great in either sect, Diurnal CU is still miles ahead of Noct CU. It might be an awkward tool to make use of, but at least it's something.