Check this Reddit thread for tested information on WHM and AST stats: https://www.reddit.com/r/ffxiv/comments/54f2hu/whm_and_diurnal_ast_healing_formulas_and_more

Crit is the most powerful boost for your direct casts as well as your HoTs and DoTs, so that should always be prioritised over SpS and Det. SpS and Det give about equal boosts, but for reasons explained in that thread, SpS goes ahead Det in longer fights. But both SpS and Det are worth much less than crit.

Because SCH also values crit over all other secondary stats, the priorities are currently same for all healer jobs.

If you're planning to raid, you should also aim for capped accuracy (currently 592 I think) and enough Pie (MP) to be comfortable.