There are two things an aftermarket "lowering" spring changes from stock. #1 It lowers the car. #2 The stiffness rate is higher. #1 will (almost) allows help the handling of your vehicle, it really depends on what kind of racing you do, but all in all, a lower center of gravity makes great changes. When it comes to having a stiffer suspension.. well.. that depends on you, and it makes you work a bit harder. The reason it depends on you is because you have to adapt to driving it with the different suspension and it will always physically and mentally wear out down faster than driving with a softer setup. A stiffer spring doesn't necessarily mean your car will handle better. After a point, if the roads you are riding on aren't smooth, you would handle better with a softer suspension. This would apply to both springs/shocks and coilovers, it just depends how different they are from stock.