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. .
I started a thread on that subject a while back:
http://www.gangsterbb.net/threads/ubbthreads.php?ubb=showflat&Number=444618&Searchpage=1&Main=16277&Words=%2Bsilver+%2Bplatter&Search=true#Post444618