If dual-fleeting, you are (in my humble view) better having two aircraft which are so distinctly different that you run a much lower risk of falling into the familiarity traps set by either. The flightdeck layouts either need to be bang identical or way removed from each other - subtle differences set a whole load of human factors traps to be uncovered at one's peril further down the line. Others may take a different view, but that's the opinion I've reached from seeing a couple of these experiments (both good and bad) over time.
It is surprising that Britannia basically down-spec'ed the later 737-200s to keep commonality with the older variants. I always remember one senior chap regaling me with the story that for the 10+ years where Britannia's fleet exclusively comprised 737-200s, the Flight Ops department had invented ten new ways (one a year) to fly the -200 to justify the department's continued scale and existence. The arrival of the 767s must have been a godsend in terms of putting a stop to the SOP "development" on the 737 fleet.