IMHO the major difference is that Michael sending Pentangeli to a sitdown with the Rosatos was actually believable. But I always found it incredibly stupid that Vito would have sent Luca to act like he was unhappy with the family, given that everyone in the underworld would have known how loyal Luca Brasi was to Don Corleone. Even with the pretext of wanting a piece of the drug money and going behind Vito's back, thinking that enemies who might be planning a hit would accept that Luca wasn't happy is stupid, since even if he WAS unhappy, what would happen the moment Sollozzo's goons killed Vito? Luca would be after them. Ridiculous.

Pentangeli could/did have the impression that he might not like the Rosatos but "the boss" was making him play nice. That's way more believable given the circumstances and probably would have worked. But I do agree that Pentangeli was expendable.