The springs lower the car. That's is. Coil overs are high performance "shocks" so you aren't just lowering the car, but have better shocks for performance reasons. Many are adjustable so you can increase firmness, or decrease it. You can adjust ride height, etc. Coilovers are performance shocks and will improve handling, cornering, etc. Springs just lower the car, and are a fixed amount. If you want to lower the car and get off cheap, coils are a cheap easy way to fix it. If you want to improve the handling of the car, or adjust it, coilovers is the more expensive way to go.
07 3.5L Charger K&N CAI, Magnaflow Cat back, JBA shorty's, DS 93 tune, sold, 2010 Challenger R/T 5 speed. 180 degree Thermostat, fan settings lowered, CC, ceramic brake pads, slotted rotors, red and black caliper paint, Challenger label for calipers, Corsa Cat back, functional hood ducts, and hood struts, 93 DiabloSport tune, Mopar drop in filter, JBA Shorties, Wideband O2, custome tweaked tune, Anti-sway bars front and rear, end links, strut brace, 12" subwoofer with amp.