Considered BiS for WHM?

[Slark]

I personally prefer Crit > Det > SS. Crit may not effect stoneskin, but it does effect regen, which is extremely useful. In my opinion, crit builds are the most noticeable ones. Mainly because both spell speed and det's returns are near worthless in my opinion. Having an extra 13% or more crit is a very big deal when you are spamming cure1, or regen/medica2 is ticking on possibly multiple people at once.

No encounter really requires insane constant HPS for a long enough period where DET can really shine. However, tons of encounters have big spikes of damage where crits come in handy.
Spell speeds bonus's are so small, and they will most likely get completely nullified when server/computer lag is taken into account. Also, a huge part of WHM healing is precasting cures, which when done, will cause spell speed to be 100% useless.

Some may argue that "crits are unpredictable," or that "crits only overheal," but having a crit heal during a death sentence or on double dreadnaught healing is essentially the same thing as using benediction because it allows you to completely recover from a tight spot and allow you to catch up without blowing CDs or spending a large amount of mana, and when you are spamming those heals, crits have a good chance of happening. Crits can potentially save me hundreds, if not thousands of mana per bossfight. Spell speed theoretically makes you run OOM faster, and DET's increased HPS isn't really noticeable on burst abilities like table flip or death sentence.

Also, when you are in mostly Ilvl90, or full, you will notice that regardless if you have a DET build or not, your healing numbers are well above whats required for anything. So much so, that i specced out of 30 MND in favor of Piety because it allows me to be more lax with mana and rely on ballad less often, if ever. (Also, 5k+ mana pool OP)

WHM's don't really have enmity issues now that tank aggro has been fixed, so that argument is out of the window.
It's also good to point out that crit has the best scaling in regards to Divine Seal/Fey Illumination/Convalescence. Divine seal+regen has a huge uptime, and from what i remember, my regens can crit for around 1000-1100 health with divineseal+convalesence, With pretty much a DET build, without 30MND from attributes. Take that for whatever you think it's worth.
In regards to crits overhealing: If you are crittng, and those heals are overhealing, odds are very high that increase in spell speed or det is also not doing much for you. Sure, crits pull more aggro than the other two options in this scenario, but enmity is pretty negligible at this point.

At the end of the day, all secondary stats are completely underwhelming for WHM, and none of them effect your playstyle really. If you are relying on crits to proc, saving tanks because you are technically allowed to act 0.2 seconds faster, or because you heal for 30-50 more health, something is wrong, and in those situations, i almost always have popped benediction, causing any of the three to be rather negligible, or completely useless. In my opinion, you should just get whatever ilvl90 you can get your hands on. If you are looking to level another class, I would just start spending mythology on that new class regardless of what stats your ilvl90 whm gear gives as opposed to trying to optimize for a single stat and replace ilvl90 gear that doesn't have the stat you want. Just get whatever you prefer playstyle wise if you are hellbent on customizing your whm's gear.

Differences:
Full crit build: 13% more crit or so, capping you at ~18.85% crit
Full det build: 30 more hp on cure 1, 50 more hp on cure 2 per cast.
Full SS build: ~0.2 seconds faster on your spells, and ~0.15 seconds faster on GCD.

Make your choice, you won't notice a big difference.

Also, if any WHM secondary stat gets buffed to the point where we are supposed to stack one similarly to how SCH stacks crit, IMO it will probably be spell speed if that means anything to you.
One more thing: SS builds let you use the iconic WHM myth robe, where as DET/CRIT lets you use the allagan robe instead. That could be a pretty big significance to alot of people as well. Also, Thyrus clips through WHM hood, which is really annoying.