RB will not touch exported playlists on its own. You have to tell it a specific playlist to export.
in RB, if you select a playlist and then export it to USB and that playlist is already there, it will first warn you that the playlist exists, and if you want to overwrite it. If you say ok, it will then roll through and update that playlist and copy over any tracks that are missing.
RB will not, however, copy over the existing tracks. This can be a problem if you have updated the track's metadata...it will not touch the existing tracks (say you updated the comment field, genre, key, etc... that information will be stale on the USB)
(Can't answer the itunes point, I don't use itunes at all)