Specular maps are often grey-scale versions of the dif texture, levels slightly adjusted… Some however are inverted… Which is the best for realism? Grey-scale or Inverted Grey-scale? The same is true for bump textures as well. (I do understand why hair needs to be inverted on bumps)
Neither - both are actually BAD types of those maps. A proper specular map, for example, would have brighter colors on the forehead and nose, darker around the jaw area, and such, with pores a lot stronger dark presence than on the diffuse map. The bump map, on the other hand, should be specifically designed as such, because just desaturating the diffuse might make the pores sink in, but would also make eyebrows, facial hair, and the like sink in, while inverting might make facial hair and eyebrows pop, but will also make pores pop.
The first thing I do when starting a new character render is to remove all spec maps…I don’t use them at all these days. IMHO if the lighting is good then they are not needed. But this is just my opinion from expreience, what little I have.
Lighting can’t change the fact that the brow and nose should be shinier than the jawline (just as an example). Not using a specular map MIGHT be doable for longer range renders, but it REALLY detracts from the realism of a portrait render, unless the specular map is one of the above-mentioned “just the diffuse desaturated and played with” which are unrealistic to begin with.