Ideal spring rate will depend on your application... specifically, how bumpy of a surface are you running on, and what tires? If you have a R-compound high grip tire running on a smooth concrete surface, the lack of bumps will allow you to lower the car more than for a bumpy surface, and the combination of high-grip tires plus lowered ride height will require really, really stiff springs to keep the car from rolling over into it's bump stops. (When the suspension hits a stiff bump stop - bottoming out, or reaches it's limit of max droop travel - topping out, the spring rate rises suddenly and dramatically, and there is a corresponding drastic loss of traction at that corner.) If you are not running r-compound or race slicks, you won't need as much spring rate, as your vehicle won't generate as much cornering force and the springs won't have to resist as hard to keep the suspension off of it's bump stops. (In my case, I actually set as high a front spring rate as is needed to keep my wide r-compound rubber from rubbing on the fenders.) The ideal spring rates will give you the desired ride height yet be generally as soft as possible to allow for maximum compliance while keeping the suspension off of it's bump stops. I'm hesitant to just throw out the spring rates I'm using, as they will only be useful to someone with the same type of car with about the same weight, with similar swaybar configuation, with the same tires, driving only on the same kind of surface.
Coilovers using standard inside diameter springs allow economical, quick and easy substitution of various rate springs to make experimentation easier. If dialing the right spring rate (maximizing grip) is important to you, I would recommend coilovers.
HTH... good luck.