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Aftermarket springs (lowering springs) will work with your original equipment shocks (the ones that come with the car). The coilovers are a complete suspension swap that have both the shock and the spring as a combination. Often enough the benefits of coilovers is the ability to adjust the dampening of the shocks rebound, compression or both, as well as lower the height of the car. As well, with coilovers, many of them will come with a camber plate adjustment in the front of the car so that you could adjust the camber.
Coilovers are clearly the more expensive route compared to getting just lowering springs. It really would depend on your use of the Z. Is it a daily driver? Do you plan to take the car to the occasional track event?
Sometimes just the lowering springs are enough. That is if all you want is to lower the cars stance. However, with lowering springs you will have to change out your OEM shocks after about 10K miles sometimes more. The OEM shocks are not designed to be fit with the lowering springs per se. IT works, you can do this, but at some point the shocks will go bad because of the less stroke that they will then have with the lowering springs.
IF you have any other questions about it, feel free to PM or ask.
http://www.endura-tech.com
By the way, we will have 350Z coilovers available in March. If you are interested let me know.
Justin Endura-Tech