Springs are not adjustable and just lower the car.
Coilovers let you raise and lower the ride height and they're all stiffer than any springs that you will find. real coilovers come with integrated shocks and springs, and the good ones allow height and dampening adjustment (how stiff the shocks are). They will actually help your car handle better.
The spring perch does not control the stiffness of the ride, it's just there to let you raise the spring up higher to increase the ride height. A springs stiffness can't be altered, but shocks are adjustable.