ADVICE SOUGHT: Should I work for SEIU or GCI?

As a just-graduated BA in polisci, I've been spending the days since commencement.....that's right, on my parents' couch surfing for jobs. Things are rolling along right now, though. Phone interviews have happened or are imminent with both SEIU organizers and Grassroots Campaigns, Inc. (GCI) staffers. So this is an open call for advice, ideally from those who have worked for one or even both of these organizations.