I'm in the same situation with essentially the same goals and considerations. Both schools are sufficiently reputable for my purposes. However, I want to work in the foreign service as soon as possible rather than "eventually", so the relative prestige of SAIS and its alumni connections don't sway me. I'm tempted to say that in our position choosing SAIS over Elliott would be solely an act of vanity--unless, of course, you're aiming for the economic cone.