US Armed Forces - Americas
If you're only going to get one, get Britannia. It takes the ideas of The East and runs with them, it plays 2-5, and (if you have an older copy of HT) has functional two-player rules that can easily be ported to the base game.
That said, why not get both? The East is still pretty great; Britannia is just better.
What he said. But if you love the game get both. Britannia is a bit better for 2 player. But I enjoy them both.