I think everyone is forgetting that when the decision to make Sim was made, Phlox thought he could harvest the parts needed without killing Sim. So they would have Trip and a copy of Trip with a short life span running around. After Sim grew up Phlox realized the procedure would kill Sim. I don't think the decision to make Sim was morally wrong, it's no different than parents who have another child to use the stem cells from the placenta to cure an already sick child they have. What unfortunately happened was that it turned out the surgery would kill Sim. That's when the dilemma came in. Well, and the dilemma that Sim only had a 15 day life span, but whose to say that's better or worse than being a slug in a jar.
(Not going to even touch Tuvix.)
^^ This, all of this, I agree. You covered everything I was thinking to mention as I read through this thread.
I only saw "Tuvix" once. The ending made me very uncomfortable, and I've not sought it out to watch since. I've seen "Similitude" more times than I can count, and loved it every time. Both episodes have that classic-Trek "impossible dilemma" storyline, but "Similitude" drew me in without putting me off--perhaps because Sim chose to sacrifice his life to save Trip and help save the world. And because you could literally see the toll the situation was taking on Archer, as he grew more haggard with each passing day. And Phlox being the surrogate father who has to perform the surgery on his "son." And Sim's unfettered, unrequited feelings for T'Pol. And Sim petting Porthos at the end. It just destroyed me. Beautifully done, heartbreaking. One of my top 3 episodes of Enterprise