You should use Flamethrower for two reasons. For one, it's higher powered, and though the 10 BP difference may seem insignificant, it can be enough to get an OHKO instead of a 2HKO. The second reason, as Soul Stealer said, is for the lower burn chance. If you have Yawn, I can only assume you want to be able to put opponents to sleep, which you cannot do if they're burned.
Hope I helped!