I thought he shot McClusky an extra time because he missed the previous shot and got him in the throat, while Sollozzo he got clean in the head.
As we do sometimes, we consult the novel. About McCluskey's murder: "This shot was bad, not mortal."
I think that we were told he had to get Sollozo first, because that man was younger, faster and would retaliate if given a chance. McCluskey was old and slow, so Michael could take a bit more time with him.
