Dave, I must admit my ignorance. I didn't know there was fire -rated mdf. Although, it makes sense, just didn't cross my mind. I don't know the price difference, but suspect that even if I did, I would have gone the drywall route.
I like the idea of having a backer to glue the mdf to, in conjunction with screws driven into the studs. Seems to me to provide an even more stable wall. I really didn't want the seams to pop after installation.
Also, I did all of the work myself, so labor costs were not the primary consideration.(quite the opposite from my day job, where labor costs rule the day)
Dan