Definitions

Niche: The Term Niche, is that given to a audience, or genre specific product, in this case magazines. A niche magazine shares the same ideologies and has a small audience that is specific.
Financially Niche magazines sell less magazines, but have less funding into them, and in multiple cases are published far less frequently apposed to mainstream, for example fRoots magazine, is only published 9 times a year.

Mainsteam: Is the term given to a mass audience targetting magazine, which is frequently published, and has large funding resources, from which it can produce more frequent, high spec magazines.
Kerrang! would be a prime example of a magazine of a mainstream magazine, as 52,272 copies are sold each issue, and 26 issues are sold a year, or Q which sells 103,017 copies per issue, but only 6 issues a year.