Take in consideration that the ground services offered by TG at BKK are superior to LX F. You will have the buggy taking you directly to immigrations/customs or to the TG Royal First Lounge if you want to shower/eat/rest before going to immigrations. With LX, you won't have that.
Having just flown TG F a few weeks ago ex-FCO, if you are on the old 747s with new F class from MUC, I would take the TG flight. If it is a newer 747 with the old F, perhaps LX would be a better inflight product.