Manning released a large volume of information to the public en masse with no regard to it's whistleblowing value. Some of that information was dangerous and harmful.
Manning's battering ram is much different than Snowden's scalpel, even though their base motivations had a lot in common. Snowden knew what he was doing and did it with great care and consideration. Manning was suffering from some serious mental health issues and did something foolish with questionable value at best.