Not familiar with Manganiello, but I looked at his IMDb photo gallery, and I don't think he looks as much like Superman as Cavill does. His features are a bit too rough-hewn. Also, his nose looks a little crooked, as if it's been broken, which is unlikely for Clark Kent. Plus he has brown eyes instead of blue.
If he had been cast, though, he would've broken Christopher Reeve's record as the tallest screen Superman.