The defense doesn't get to call witnesses until the trial. This is the equivalent of a grand jury - where the prosecutor makes his case for indictment. The defense never participates in a grand jury.
There are some differences, for sure - a grand jury almost always meets in secret, and any potential subjects of indictment are not publicly announced until after the grand jury has approved indictment. A grand jury is also meant to be impartial, which is an impossibility with impeachment (where the grand jury is composed of the House). However, in terms of outcome, this is the same thing. Impeachment is an indictment, not a conviction.
