Paul - you need both types if you are to have a split system.
Leisure battery is designed to have a small drain over a long period - ie - run electronics at anchor.
Car battery - this is a cranking battery and what you will use to start the engine, designed for very short and high drain\amp applications.
Then once started on the car battery (switch position 1) then the switch put in position 1+2 to charge both when under way. Then when at anchor switch poition 2 is used.
I am sure a relay automatic switching system is possible but the 1, 2, 1+2, 0 position switch is cheaper!
Rob