α-Halo ketone and strong base: Favorskii vs SN2
I would comment however I am new and have yet to unlock that feature. 

Do you have an exact substrate of interest? I have studied the Favorskii rearrangement a tiny bit in a graduate mechanisms course and if your seeking a very general answer to a general question; The favorskii seems to be favorable when your substrate is containing rings that are conformationally rigid where our alpha-halo-ketone moiety is found.

Check out Eaton's 1964 synthesis of Cubane to find two really elegant Favorskii reactions in the synthesis of Cubane. Notice for almost all of our examples of favorskii reactivity we notice a common pattern in the substrate scope? 

Rings! My theory is conformational rigidity of a ring is partially responsible and we can confidently point our fingers to Favorskii where cyclopropanone intermediates are possible AND energetically favorable (conformation argument). 

If our substrate scope is including non-ring alpha-halo-ketones.. We may want to consider alternatives like SN2 to predict reactivity.