Would W really want to reinstate the Batista thugs and cronies from Miami in power?
That would require an occupation of Cuba to accomplish. As it stands, if Fidel was killed at this point in time, Raul would just take over, Cuba would condemn the United States, and relations with the rest of Latin America would suffer as it would look like the US was reigniting an old conflict for no real reason (and incidentally that would have been true).
Even if this were successful (and Castro has survived an unholy amount of assassination attempts), what gain is there really? Even if the US completely covered its tracks, it's not like the list of suspects will be all that long, the Cuban government and its people would blame the evil Yankee imperialists and the current trend of warming Cuban-American relations would be rendered politically impossible.
While the Castro brothers are a major part of Cuban politics, there are plenty of people who are going to be able to take charge if either Fidel or Raul or both are killed.
Bush
liking the idea of being known as the American president who took out Castro is not at all outside the realm of possibility, but the cost of such an action would be enormous, and this is assuming that things don't go full-retard and lead to an extremely rough occupation of Cuba and subsequent guerrilla warfare. The Cuban government and people have lived under the shadow of the American military for years, is anyone at this point really assuming the Cubans don't have plans for a long-term and extremely brutal guerrilla war against an American occupation?