If you are interested in learning about discovery issues, the magistrate judge would be great. While it may seem boring, it is certainly a helpful experience since that's what attorneys spend most of their time doing.
If you looking for an analytical writing sample, both the US Attorneys and the District Court will serve this purpose.
I think that all three options are prestigous and you should base your decision which chambers or office you feel most comfortable w/. This is important, seeing that for some judges you may have to make an appointment just to discuss your bench memo.
If you'd like to know more about the US Atty's Office, I would ask if you could stop by for an hour or two and just see what a typical morning is like. I'm not sure if a judge would be willing to do this, though.