Unless you have an defective AK that has an issue that can be corrected with a stronger spring, use a standard weight.
People insist on installing stronger springs in military designed weapons "Just because", and so they can make it "work better".
Like all military weapons the AK was carefully engineered to use a very specific spring and installing something else often makes problems where there were none.
So, unless you have a very specific defect in your AK that can be worked around by using a different spring, use a factory weight spring. That's what Klashnikov spent all those years of development deciding it needed to have.