I think SOnny and Michael had more similarites than people think.

They are both cunning. Sonny thinks very logically after he is told his father has been shot, he writes information on the wall, and does NOT lose his rag with Sollozzo when he phones him. He knows Paulie Gatto is responsible, possibly Clemenza too. So he logically uses Tessio's men only. The only time Sonny gets mad if is a family member is directly affected. Same for Mike...

Sonny hits Bruno for what they did to Vito...
Michael hits Carlo for what they did to Sonny...
Sonny beats the crap out of Carlo for smacking his sister about, only stopping short of killing him becuase his sister is pregnant.
Michael hits Moe Greene for smacking Fredo about in public (and to help close the casino deal - obviously)

Michael's temper...while clearly not as physically violent as Sonny's, is there very much there, he loses it with both Tom and Kay during part II and gets very annoyed with Vincent throughtout part III.

The pair also agreed that killing Sollozzo was the only way forward against Tom's better judgement.

Sonny if he just learned to calm that temper would have been a great and loyal Don. He aslo would not have alienated Fredo as he was his elder and therefore not ordering his older bro about. Fredo worshipped Sonny as is evidenced from the flashback scene at the end of GFII. Sonny would have trusted Freddie to handle a wing of the family. His confidence was up from Las Vegas.