Yeah, SpS is a very bad suggestion. This is not WoW where you have to anticipate attacks and then react quickly. Every fight in this game is choreographed -- you know exactly what is coming and when. Additionally, the amount of SpS you need to reach any useful breakpoint is massive, and SpS does NOT effect H/DoTs. If your you ever say that SpS "saved" your tank, it's because you, the healer, did not know how to do the fight, did not know what abilities were coming, or did not time spells between auto-attacks. Essentially, you failed at knowing what you were doing, and should review your play footage to improve.
Crit is not optimal as this offers uncontrollable bursts of healing output and nothing that WHM has depends on Crit.
Determination on the other hand offers a consistent increase in your throughput. It is the best stat to focus on, period.