The two terms are used intermixedly (izzat a word?), meaning, depending on who is using it, depends on if they mean the same thing or if they have different meanings.
Example:
Telsa. I would NEVER put them in either category. Just because they have long hair (like most rock stars dating back to the 60's and definitely from the 70's on forward), but that doesn't make them hair metal. Metal is even stretching it with Tesla. More like straight up, in your face hard rock, but they OFTEN get listed as being hair metal
Back in 1985-87, there were a lot of hard rock acts that took up a "glam" look. Kiss, Motley Crue, Twisted Sister, Cinderella, Poison. Ratt, Hanoi Rocks, Quiet Riot, hell, even Aerosmith to an extent (although I'd never lump them in with any metal), Faster Pussycat, Stryper, WASP, Britny Fox, etc etc etc.
Musically, I wouldn't put those bands all in the same category and i'm not sure they deserve to be sectioned off from other bands based soley on picture on the front of back of their album, or because of clothes that they wore on 1 or 2 tours. *shrug* .
So, to answer the question, I don't have an answer. There are negative connotations by some people for either term. I personally don't care and I embrace the vast majority of the music as many of my all-time favorite bands went through a few years or it or lived and died by those two sub-genres. Bottom line, I think that both terms are more appearance based than musical style based and to me, the music matters FAR more than the "look" of the day. 80's bands wore makeup? Oh noez! (hint, so did bands in the 70's and 90's)