Coilovers will give you the ability to tune your ride to exactly how you want it. You can dial in the exact ride height you want. Then how stiff or soft you want your dampers to be. Secondly if you want to change something at a later date you can.
With springs only changed you are stuck with what height the manufacturer suggests and you are stuck with factory dampers which are always a compromise. I have had and am currently deciding what coilovers to go on the F10. KWV3, Bilstein, Megan or BC. ULTIMATELY MY CHOICE WILL COME DOWN TO PRICE, EASE OF ADJUSTABILITY.