[Alan l.]

Your suspension consists of two key parts.

Shocks and Springs

What most people do is change out just the springs while keeping the factory shock to reduce the ride height. This will impact your handling and ride comfort in one way or another. The plus side of doing just the springs is that its relatively low cost (250-600) and you get to keep some of the bmw ride comfort. The negative side is that your oem shocks wasn't designed around these springs and could have a bad affect on your ride quality, handling and shock longevity. As long as your springs are from a legit company this usually isn't an issue. You are limited to dropping your car between 1-1.5" max with just springs. Any lower your shock won't be able to handle it and will wear out much faster than they are suppose to. 1-1.5" is enough for most people but it won't eliminate ALL the gap for sure.

Coilovers = Shock and spring combo

You are getting a combination of the shock and spring with coilovers. You can also adjust the height to whatever you want as well as the dampening settings. Plus side is that you get almost unlimited adjustability of your suspension while the down side is much higher cost for a reputable brand (2-3K for KW) and not everyone will know how to reach the optimum setting for their car on their own. Keep in mind that coilovers are designed to work on multiple platforms so they are not tested or dialed in for the F10 like your factory stuff are. Unless you plan on tracking your F10 or want to slam the car to the ground I don't think coilovers are needed.

If you are going for a moderate drop i would highly suggest you go with ACS springs. (eibach, H&R, RDsport are options too). I've seen numerous threads here where people usually swap out other springs to ACS and loves them. When it comes to suspension its all about Comfort, Looks and Handling. You are usually sacrificing something for the other and the key is to find the right balance of the three for a daily driver.

Alan