Another interesting question would be, when does it make sense to do mixed fleet flying? Does it depend on the fleet size?
Are there values known when it would be feasible?
A330/340 MFF is a clearer case, and is done by most airlines flying both types.
But A320/330 MFF is done by some, not by others.
For example Qatar don't do any MFF
afaik, but Etihad do.