I don't think Sonny would have made the deal for several reasons. First, as Oli noted, was his temperament. Second, he realized (per the novel) that when he shot his mouth off at the meeting, he exposed his greed and unintentionally encouraged Sol's attempt on Vito's life. Vito already knew that Tom and Sonny were for the drugs deal. I believe the reason he had Sonny, Tom, Fredo,Tess and Clem present at the meeting was because he wanted everyone to hear that his "no" was final--and wanted Sol to see that everyone heard that final "no" to keep him from trying an end-run around Vito. But, Sonny's outburst became (in Puzo's words), the c***k in the Don's armor." Sonny may have believed that if he'd said yes to the deal, he might have been perceived as benefiting from Vito's death by becoming Don and getting rich from drugs.

Tom's advice was rational, realistic, conciliatory, peaceable--the work of the lawyer that he was. But, he wasn't a Sicilian or a Mafioso. If he were, he would have seen that, if Sonny agreed to the drugs deal, he would have fatally weakened the Corleones by allowing an outsider--not even a made man in the US Mafia--get away with killing the great Vito Corleone. Sonny wouldn't have been the Don that Vito was. But, he was a feared figure in the Mob world, and (again as the novel points out), a superb street general. Fear counts for a lot in the Mob world--that's why Michael never gave anybody a pass, not even his own brother. If Sonny followed Tom's advice, he'd be giving Sol a pass that wouldn't escape notice by the other families, even if he eventually killed Sol.