I don't think that it is a double standard because these decisions affect entirely different environments. In the case of Quinn, his decision was (as many have said) a personal one and any effect that it would have on the Continuum would be due to subjective opinions of other Qs.
In the case of Tuvix, he was the combination of two critical officers of Voyager's crew. Though Tuvix displayed a vast majority of those skills, he could not perform them at the proficency of the original officers. Tuvix was made of both Neelix and Tuvok, and to keep him alive would be to condemn two officers to certain death. Why should Janeway allow that if there is a chance to save them? Yes Tuvix is a life form himself, but so are Neelix and Tuvok.