I'm not sure any good or healthy decision has ever been made as a result of an ultimatum.
What you probably need is some time and distance from both of your..."options"...to figure yourself out, how you got here, and what is best for everyone moving forward. Yes, it is a very real possibility that you might lose both of them in the interim. That is a consequence of your choices thusfar, and it might be what is best for everyone. Certainly, everyone gets to decide that for themselves except the children, and it's possible that what is best for these children are the people who are willing to put their interest and their emotional health first. Unfortunately, in states of high drama such as this, people don't always really understand what that means.
No one here can give you odds on Sarah's recovery. And I very much doubt that anyone is going to offer an opinion on who to choose. The situation is extremely unhealthy for everyone and I hope that it resolves in the best interest of the kids.