It's the difference between fighting the Zerg, and fighting the nazis. They both genocide you if they win, but it feels like a different fight. They also stem from different ethics. The hunger would probably be fanatic collectivists, where purifiers are usually xenophobe spiritualists
Collectivists will be limited to Displacement only Purges. And caste System Slavery, wich is not suiteable for lifestocking.
According to our current information, only Xenophobes are able to eat people - both Lifestock and processing. The others consider them "to smart" to just eat them.
Only real difference is how they would react to you doing the same. There is already precedent for that:
Slavers do not care if you enslave.*
Purgers not if you Purge.*
Dominators not if you conquer Pre-FTL's.
*With overrides for "their" species, of course.
Unlike the fanatical purifiers, these guys are quite diplomatic and even ensure that their cattle are raised in utopian conditions.
Who wants to fly half way across the galaxy for low quality meat anyways? *Kanamits quality guarantee, taste the difference!*
How about this one? I think we could do it with advanced Genetic Engineering: