SSD is the more valuable ability so I would prioritize using it first and getting it on cool down before DFD. Under rare circumstances should you use SSD without BOTD, which makes its potency 221 on a 60 second cd and DFD is 250 on a 120s cd.

The exception is if there's some sort of AOE phase that would somehow be affected if you delay DFD a gcd or two.