I was debating the same things for quite a while and decided to just but a used set of coilovers for $700 instead of H&R sport springs. Reason being, I heard the ride quality is not that great on H&R sport springs (unless you pair them with koni yellows), and I would prefer the adjustability of the coilovers. Also H&R sport springs + koni yellows are almost the same price of used coilovers
Check out BC racing coilovers: XTR performance sells them and you can get them here to: http://www.flushstance.com/bcracing-coilovers/bmw/
They give a very good drop and come with front camber plates, if you want to have the ability to slam your car whenever you want in the future buy the BR Type- Extreme drop version.
Hope this helps!