Manning and Snowden are simply not comparable at all.
Snowden for his one personal beliefs about the legality and morality of the situation acted. Yes he did violate the law but that was after he attempted to use the proper channels and was shut down. When he finally did go to the press/public he made arrangements to filter, redact, and limit the release of the material with the help of a few trusted press agents. It was of course necessary to disclose some secrets because without doing so the public/press would have little to no way to affirm the credibility of anything he was saying about the existence of the invasive domestic spying programs.
Manning was entirely different. He basically was talked into doing what he did, and had help of an external actor. His reasons appear to be more born out of a desire to personally get even with 'the system' than to be a reformer. He made no effort to use the proper channels that we know of and little effort to control the release of material. He certainly took no personal responsibility for material handing it all to wikileaks with no judgement of his own about what was or was not to harmful to leak.
Finally Snowden's leaks might have harmed intelligence gathering efforts, disclosing methods and capabilities but they did not out people as Manning's leaks almost certainly did. There is cause to believe lives may have been lost due to Manning's leaks. That issue alone should make it a very different discussion about pardoning him, and the moral justification for his actions.
I can see a pardon or reduced sentence for Snowden but there is no way I would ever let Manning out of the clink.