For a jam the old input system does the job well. Just use GetAxis for movement, it's easier to use and has better compatibility (works with gamepad and non qwerty keyboards).
The new input system is probably better for big projects with complex inputs and with support to a lot of different devices. It's easier to customize and can deal with more complex inputs than just pressing and releasing keys. But it's also a bit more complex to use so it's not necessarily better for a small game.