efficacy or swift affliction?

which is better

Uuh...they don't do the same thing.

Depends. We need more context here, vs bosses, vs monster packs, skill you want to have it used with etc.

Efficacy is better for Scorching Rays since both compoment affect Scorching Ray.

Efficacy is a support purpose-built for skills with an integral DoT component, preferably spells that scale the dot, like Scorching ray or, even better, Essence Drain, which profits from all three components, in effect multiplicatively.
For skills that do so benefit, efficacy is clearly stronger.
If you focus on the dot of a skill that doesn't scale from base damage, or isn't a spell, swift affliction is logically better.