It is really a question of how you like a car setup as well.
I know from the days when Jim Ricghards and Mark Skaife were together, MS liked the back end of his car setup so that it was soft in the rear which would give very good power down pout of the corner, but would easily pick up the inside front wheel.
My RX2 was setup like this, and it had 400lb/in fronts (a little hard) and 200lb/in rears and was used for cct work only.
Get your current ones measured and give the harder ones a go and see what feels better as well as compare lap times.
Don't forget that if you change the spring rates and keep the same heights, your dynamic roll centres will change as well.