Sim only chose death because the alternative was a few more days of life and then death.
The moral dilemma in Similitude to me wasn't killing him so much as creating him. Creating a sentient life form for a strictly utilitarian purpose. And it's something they probably wouldn't have done if they weren't in a situation where Trip's survival didn't significantly increase the odds of saving Earth.
So the difference between Tuvix and Similitude is the magnitude of the stakes.