By providing a nice form with a drop-down, you can avoid worrying about the user=foo string on the URL. This has a minor advantage that a typical end-user may not be curious about the user=foo and play around with it (by typing other id's).

I would tend to avoid having an end user worry at all about query_string data, i.e. having them fill in form variables in the browser's "Location" entry field, and make them use widgets you supply, like in this case, a simple popup menu.