I have also been accepted to GW, but placed on the alternate list for funding. GW is my first choice, and from that perspective, I would prefer if you rejected the offer.
At the same time, I am curious to know, if you don't mind sharing, what's your subfield and what particular professors you're interested in working with? GW has people like Barnett and Finnemore, who are (with Wendt, Blyth and a few other people the leading constructivist scholars). I definitely think this is a good sign, especially as I am interested in working with them on issues of global governance and international security. And also, for policy-related jobs, GW is definitely a top-tier program. By when do you think you'll decide?