Upon being liberated from the Borg, Annika wanted to return. Now you could argue at that time she wasn't in a fit mental state to make an informed choice. If 4 years later, she made the choice, then it might be considered an informed choice. Would Janeway have the right then to stop her?
Tuvix appeared to be making an informed choice.
It wasn't even an issue Janeway should have delt with, she could have easily put him into stasis until such time as an uninvested party could rule on the dilema.