coilover sleeves have shot springs and adjustable perch/ring on a sleeve about 3-4 inches...you can adjust the thread on the sleeve to raise/lower the car. The negative side of this is the spring has free play...like when you jack up the car you can move the spring up and down with your hand...there is room for "play" springs are always compressed wether it be on the ground or jacked up...they are always pushing the wheel down to the ground. they generally have lower stiffness or spring rates than coilover springs which are very small in size but thick in general.
true coilovers have threaded shock body's and generally longer springs with a helper spring which makes them the most desirable.
my suggestion is to get neuspeed sport or H&R sport(I like the best) if you decide on keeping your stock shocks and need clearence.
if you really want to lower it and get the best handling possible get race springs by neuspeed or H&R....I like these two spring companies the best!