I have to say that I don´t see the drawback of spending a year in Italy and then your second year in DC with SAIS. I too am in a similar position, having received substantial scholarships to attend both SAIS (Bologna, which I chose in my application) and SIPA. I agree with CharlotteAsia that there is nothing less prestigious about doing the Bologna program at SAIS. Indeed, what employer wouldn´t look at a year studying abroad at the top international affairs grad school in the country as something to be valued? Moreover, you would be taking many of the same classes in DC as you do in Bologna--that is, the core classes that are required for the international economics specialization. The think tanks and other orgs in DC will still be there when you get there for your second year. You could even get an internship in DC for your first summer if you desire.
Maybe It´s because I´m American and really value the opportunity to study/live abroad whenever possible (especially in Italy!), but JHU SAIS in Bologna seems like a no brainer over SIPA, even more so if they are giving you $30k vs. nothing at Columbia!!!