If we're talking immunes, "-x to enemy resistance" doesn't apply unless the immunity is first broken by Conviction aura or Lower Resist curse. And as others have said, it only affects the wielder of the equipment in question. You'd have to be holding Obedience yourself to get the - res benefit on your attacks.
Fire isn't a great element to rely upon in hell, as even with Infinity merc a number of fire immunities won't be broken.
That being said, yeah if you're looking to reduce enemy res, Infinity all the way. Obedience is a great budget option for the damage and crushing blow though.