input_system::axis choice

input_system::axis is faster than input_system::axis2 when keys are not pressed and released often but are checked often. For example movement input (state check every frame). On the other hand input_system::axis2 uses less memory.

You can prefer input_system::axis_state() over input_system::axis2, because is more memory efficient, but less convenient to write.

Differences in performance and memory are marginal due to very few instances being present on the scene at one moment. So pick whichever class / method suits you.