Go for Americans - it's obviously easier to sell to American people. Band of Brothers was quite a popular TV show, and of course Saving Private Ryan was a well know film, so whatever you choose, infantry or airborne, it's an easy sell.
As the posters above say, keep it simple and don't bog it down with complicated rules or things like flamethrowers.
If possible try and have a tank or vehicle on each side. In my experience, people like tanks blasting away at each other, and they have an obvious visual appeal on the table.
Good luck with your demo.