They are all neutral evil.
EDIT: I think the variant is that you get the full imp and some magic resistance. But you're trading your soul for the bargain.
Since both the PHB and MM versions are CR1 -- what's the special familiar deal, exactly? Is it something anyone with Find Familiar can use, or is it an option only for Warlocks who can already have an Imp familiar (via Pact of the Chain)?