There are other "what ifs" to consider as well:
If you are flying a quite new type, or a type with a quite new engine, there is a real risk that a single AD will ground your entire fleet. There is some safety in having at least two types of aircraft.
If you anticipate a growing market, it may make sense to lease a smaller a/c until the market grows into your "A-fleet".
But make no mistake about it - there is a definite economy in standardization. Ask Southwest!