First, I feel that Snowden should actually have his day in court and present his case before anything related to a pardon or commutation is discussed. The American people need to see and hear both his and the government's position and evidence in a more balanced, less sensational environment than the MSM gives us.
The only question that would be debated at trial, or on which any evidence could be presented, is whether or not Snowden stole secrets. The government has overwhelming evidence that he did, including his own repeated admission, to many people, in many forums, many of them recorded and nearly all of them perfectly admissible. There would be no arguments presented as to whether his decision was justified because it was in the public interest, because that has absolutely no bearing on his guilt under the Espionage Act. The only place that would be argued is in his lawyers' appellate pleadings.
So, a trial would do nothing to enable the public to hear the sides. The trial would consist of the government submitting into evidence many pieces of proof of Snowden's act, and Snowden's complete inability to disprove any of it. It's more likely he'd just plead guilty to avoid wasting a court's time -- and making a judge who has to sentence him angry.