1. Factors to consider

There are many factors to consider when choosing a new kart seat.
If you have had a seat in the past. Do you know the size and shape?

2. Choose a Shape

Shape does not make as much difference as the teams would have you believe, but they will be nervous of miss positioning the seat and may want you to use the shape they are used to fitting.

This is fine until the seat is not the right fit for you. An ill-fitting seat can cause a driver to damage their body or hang on the wheel, neither are good for ultimate performance.

Different shapes put in the chassis with the drivers back in exactly the same place, made with the same materials will pretty much handle the same.
That said a significant angle change of the back will alter the handling especially for bigger driver and this should be looked at separately.