We always do buttons as they seem to be the main "trading capital" of First competitions.
But last year as introduced the idea of Trading Cards, and they are actually a lot less expensive to create. (and in the end, they are easy to "save" in card holder books and such...and you can do things like put together scrap books of all teams at the regional, and even page with all the alliances and cometitors on a page. (we tried our best to made them for all the team at the Nationals ;-)
-Quentin