With or without coilovers? With good coilovers you can essentially adjust the height independent of spring choice.
Are you talking street stiff or track stiff here?
Most, if not all, aftermarket replacement springs are designed more for lowering than customized spring rate and ads tend to reflect the drop amount more than the rate. They're typically also progressive rate which makes it a little more difficult to quantify the spring rate. Generally with this type of spring, the lower the stiffer.
04 IR 330i ZHP | ESS TS2+ SC | MagnaFlow "headers" | KMS 3.64 LSD | AST 4100 550/650| APEX ARC-8 18x9 | H&R front sway bar | Vorshlag camber plates, RTAB limiters | TMS subframe & swaybar reinforcements | UUC SSK & DSSR | UUC TM & TME | BW motor mounts | Dinan strut & shock tower bars | vaio 4-bolt oil pump