hey marwanslm,
I'm pretty much in the same situation, debating between Elliott & IR/PS. A huge factor for me is that Elliott gave me $0 and IR/PS gave me a full ride plus stipend. I have only heard great things about both programs, but if all things were equal I would have to say the DC locale for Elliott is a big draw. But the funding has really tipped everything for me and now I'm leaning heavily toward IR/PS. I did my undergrad at UCSD and know it's an amazing school. However, I'm still going to the Elliott Admit Day to see if I can be swayed!
I think if you're sincerely interested in a PhD, UCSD is the way to go. The poli sci department is top notch and consistently ranked among the best.I was a poli sci major and had some really great courses and professors.