The main difference between Monarch and Britannia in this regard is that this was Monarch's first foray into jet operations whereas Britannia had already racked up three years 'experience with new aircraft. With the benefit of hindsight (i.e. knowing the ABC's days were numbered), Britannia may have been better off with new Advanced 727-200s, in order to reach the distant spots that the 737s couldn't quite manage. Of course, we shall never know.