Basically... Go with whatever. They all end up being close enough for BLM.

Typically, people prioritize SpS at least up until a certain level where they personally feel the most comfortable, then they stack Crit/DH.

Others like going full SpS or full Crit.

It's worth noting that if you aren't running with a Crit focused team composition (BRD/DRG/SCH) then DH can pump out more damage than Crit.

But really, get SpS somewhere around 1500-1900 and see how you feel, if you want more SpS or more Crit/DH. Would be the easiest solution, lets you tailor to what fits your playstyle more (Which will have the biggest impact on DPS)