180 gram vinyl = better sound quality is a myth.
the usual reason given is that a deeper cut groove allows for better frequency range/response, which is true to an extent, but the thing is that the grooves are cut on the master lacquer way before a record goes to press, so the grooves on the stamper (which is being pressed into the 180 gram vinyl) are identical to what's on the master lacquer. i've never heard of a mastering engineer being told to cut specifically "for 180 gram vinyl."
180 gram is purely a cosmetic/aesthetic/marketing thing.
read here for some other good info - http://www.recordpressing.com/2010/03/t ... d-myths-2/