The difference with "The Enemy Within" is that the divided Kirks were dying, just as the alien dog did. They needed to be reintegrated in order to survive at all. So there wasn't any choice there.
Oh, my apologies, I don't remember that detail. I had thought the dog had died solely from the shock of reintegration.
But as I said, the issue with Tuvix is that he acted selfishly. He placed his own survival above that of others, and even took aggressive action to try to preserve his life.
Others who were already "dead". To paraphrase Picard, "they were already dead... what more could have happened to them?"
We'll probably have to agree to disagree about Tuvix.