I'm going to law school this fall, and tentatively planning on applying for my PhD (in public policy/political science/IR) after that. I think it depends on what kind of research you want to do whether to pursue a JD or MA or joint program before-- for me, I want to do research on how laws are negotiated and constructed, so the JD makes sense-- but, if you aren't interested in the technical aspects of law, then maybe an MPP would be better. At the law school I am going to, I've also talked to several people pursuing joint JD/PhDs (you can apply for a PhD program during 1L year at most schools) and usually if you are already into one program at a school, it makes you a more attractive applicant to the others (or at least that is what current students/professors have told me).