Good lord, Flash Fire is easily the better ability. Who cares about a 30% chance to burn with a physical hit on you? Sure, nice when it kicks in, but Flash Fire is so much better, making your Fire type even more formidable a sweeper. While you won't be using your Fire type move right away, after the foe is taken down, you can sweep through the opponent with ease. As for the foe not attacking a Pokemon with Flash Fire with a Fire type attack, that is why you switch in on the foe. Remember, the metagame is a constant battle of prediction, switching your Pokemon in and out all the time. Flash Fire works well with this, especially thanks to this Generation seeing an increase in the use of Fire types.