Terminals reduce gate cost significantly at most airports, and you're gonna have to build terminals in the future anyway if you keep expanding.
Starting ops at a second airport would help make your airline more resistant to competition and you can reuse gates at destinations that you have left over.
Either way would help you make more money, it depends on if you think you can make more money at a different airport vs expanding ops at your current home airport. If the second airport somewhat comparable in passenger numbers to your home base then starting ops at a second airport would probably make more profit, but ultimately you're gonna have to build terminals soon anyway.