The thing is chap, like many things , its all down to personal preference.
One persons ideal suspension setup maybe vastly different to the next person....I had s-line as standard and initially thought it was ok but after a while its short comings became very apparent.
I swapped to H&R 45mm springs and there was a vast improvement over the previous setup but still not as good as others.
So decided to go with Bilstein dampers aswell and it has been 1st rate, well for me that is.
Its a double sided argument really , do you change just the springs or the whole lot, ultimately depends on what you are expecting to achieve from the finished upgrade.
Is it comfort and lower ride height or performance handling and lower ride height ( those two go together anyway) you want, cost will then be a factor .
keep in mind that springs control the ride height and support etc , dampers control the important functions , damping rebound etc all are vital to getting the ride and handling you desire.
Putting shorter springs on dampers designed for longer springs may just cancel out any damping gains , spring rates vary a lot and are specific to model and variant but its only part of the suspension equation.
Maybe try a car with s-line on and then one with s-line with lowered springs and see the difference, you'll be surprised.
Coilovers are very good if you want total ride height control but when you start getting really low the spring rates increase so it kinda cancels out some of the damping making things a bit firmer as the springs are under a bit more tension.
the better the kit you get the better it will work and adjustmenst will be better, that said not all low priced kit is low for a reason as quite a bit is very good.
sorry to make things sound more complicated , but do your homework and it will pay off in the long term , you really do not want to be changing them more than once, cost and hassle...