"American" because I am an American, and it's the way I was taught. Although I have to use the word Theatre, rather than Theater because of my respect for the art and Shakespeare.
Quick question to any British people reading this: Does it matter if we call your country England, Great Britain, or the United Kingdom? I know that G.B. technically refers to England and Scotland, and U.K. refers to England, Scotland, and Ireland, but since they have been used interchangably (especially in America), should one be used over the others?