Logic puts me with Oli, Sol couldn't do business without Vito's money, judges and politicians, Vito said "no" to Sol, and his "no" was "final." He left Sol with two choices: either skulk back to Sicily with his tail between his legs, or whack Vito and try again with his successors.

BUT:

Killing a Don is an act of regicide. Sonny's show of greed had to be a decisive factor in Sol's convincing of Tattaglia to support the assassination, and in mollifying the other Dons. Sol had to have told Tatt and the other Dons: "Sonny is hot for my deal, and that Irish consigliere, Hagen, is hot for it too. With the old man out of the way, they'll go for my deal--and there will be no war." Without Sonny having opened his mouth, I'm doubtful that Sol could have persuaded a pimp like Tattaglia to support whacking Vito and risking an all-out war.