Always meld Crit first if your gear allows it, after that it just depends on what your gear is able to meld. SS or Det. Det will make your damage consistently stronger but since SS affects cast time and the overall damage of DoTs just follow this:

Crit > SS > Det

It's up to preference though. Good luck. Also, check out Ariyala gear calculator, you can see what pieces of gear can take what materia and mix and match to get a BiS.

Good luck!