Det is the bread and butter secondary for WHM. Reliable and straightforward potency increase. Crit is similar in HPS increase, but less reliable and thus not as good as det. Piety is always welcome, hard to judge it's value against crit, depends on content. Spell speed is absolute garbage unless you are really bad and start casting your heals after seeing people take damage.

Generally speaking, when comparing two equal ilvl peices, if one of them has spell speed and the other doesn't then the one without spell speed is BiS. If both have spell speed, then the one with less spell speed is BiS. There are probably some small exceptions to this rule.