Hard and smooth.
Stainless steel bearings are nice to have — for underwater applications. But if you're not planning on leaving your bottom bracket permanently submerged in water, then the smoother, more durable chromium steel bearings in FSA's MegaEVO Chrome Steel Bottom Bracket are your best bet for long running smoothness.
The MegaEVO Chrome Bottom Bracket's chromium steel bearings are housed in titanium anodized aluminum cups. It's Di2 compatible, and it also lets you enjoy the improved stiffness and lower weight of FSA's BB386EVO bottom brackets and cranks in an English (68mm) bottom bracket.