The latter is tricky. You’re creating redundancy, which is good. But that redundancy is mighty expensive, as you have to either accept a lower quality of service in both, or invest in more specialized labor for both platforms.

Analogy: One of the ways that Southwest managed to keep costs down was to use one type of plane. While its peers used a mixed group of Airbus and Boeing aircraft tailored to the route, Southwest only used the 737. This allowed it to save huge amounts of money on maintenance and pilot training, as tons of parts and crew were easily moved around. And then the 737 MAX8 clusterfuck happened, and Southwest is stuck spending more fuel than its Airbus A321-neo flying competitors are.