Sculpted hair for smaller figures because you can't' get it to scale with the figure if you use doll hair. For larger I'd go for doll hair as you can get away with it being more realistic.
For me, it mostly depends on the scale/design of the figure. On smaller figures, real hair usually doesn't look too hot, but on larger figures, like sixth-scale, it looks more realistic. Obviously, nothing looks more like hair than actual hair (no matter how well you sculpt solid hair, it's still an immobile lump at the end of the day), and the real stuff does have the benefit of not hampering neck/head articulation as well. Depending on the figure, I've used both methods, and kind of a half-way method as well (hair "fixed" into a solid form rather than "loose".