One other thing to consider (I'm going to stay out of the velocity/effectiveness discussion for now) is that many suppressor companies won't warranty their 5.56 cans when installed on a barrel less than 10.5". Not sure if that is a concern for you, I've just seen too many posts on Silencertalk from owners that bought the gun first and THEN discovered the warranty issue.

Personally, I'd go 9mm for a 7.5" and 5.56 for a 10.5".