I've used both Koni FSD's and Sports with oe zsp springs.
FSD's have better ride comfort and Sports handle better. Both are a better option than oe replacements.
I rode in a car with Bilstein HD's and they didn't feel any firmer than my Koni Sports.
As someone mentioned, if you can still get PS V1 dampers at $133, that'd be a good way to go.
2007 E90 335i, TiAg, 6AT, ZPP, ZSP, ZCW, 6FL, HD
Quaife lsd, ETS fmic, GIAC s/w, Dinan intake/exhaust/oc, KW/M3 susp, Apex Arc 8, Mich PSS