It's a challenge to believe that Sonny would have overcome his congenital instincts to pursue an intelligent course of action. However, he did do so when he told Sollozzo that he would wait.

The preponderance of advise from Tom, Clemenza, et al would have been to make the deal. They would have pointed out that Sollozzo wasn't going anywhere and he could be dealt with eventually. So, I think that Sonny would have been persuaded to make the deal. However, I don't know how Michael would have reacted.