These two situations are only the same in that they both benefitted the Republican party by misrepresenting the facts in order to make the public think the Democrats were guilty of a major political scandal.
The difference is that Comey didn't know that Congressman would do what he did, where as there is convincing evidence that Assange knew about the RNC hack and emails, possibly even had access to them and said nothing. Comey got used, but Assange knowingly helped Russians spread disinformation.