The advantages of 7-string is obviously the extended range, for metal I prefer 7-strings since I like to have the lower string, I don't like to tune down 6-stringers. So with 7-string I can have the low B (or lower) and still have the same range at the top. It's a matter of taste of course, some people rather tune down 6-stringers, but I don't like that. The biggest reason that I don't like it is that I don't want to mess with the higher strings, I want it to be a 6-stringer but with a lower string, so with that I basically only have to look at the guitar as a normal 6-string guitar with one extra low string.
I'm thinking of getting an 8-string sometime in the near future.