It's definitely unclear in BG3 but in actual D&D ceremorphosis kills the host and the mind flayer is formed from their body/mind/memories. So in Wyll's case volunteering for transformation would be a bad idea because he would immediately go to Avernus to serve Mizora forever whereas if he lives he still has time to escape his pact. BG3 says that mind flayers don't have a soul but the ones touched by the astral tadpoles do seem to retain their personalities. My interpretation is that BG3 ceremorphosis destroys the soul which is the only "true" death in D&D, so becoming a mind flayer guarantees you will never have an afterlife, which is a pretty big deal
Anyway, best choice imo is Karlach. She's dead regardless, it's not clear where her soul is going, and this way she becomes a selfless hero and continues some form of existence