Some questions to ask yourself: is there a topic you’d really like to investigate in depth? do you have the time in your schedule senior year to do this and do it well? are you planning on graduate school or a job in academia? Being able to point to a well-executed and written thesis, gives you an advantage. Also, talk with your major advisors about this.
It’s just so hard to choose one! Seriously, though, it keeps options open in more than one area, gives you the background to research issues in an interdisciplinary way, allows you to follow two separate passions, and impresses Grandma…? But don’t feel that it’s a necessity or that everyone double majors – talk with your advisor and do what’s right for you.