I am taking my parents on a car trip through Italy in a several days and I insisted on passing through Cinque terre on our way down to Rome. I knew I've made a good call.
On the OT: I don't think you can choose wrong. I've never been to Istanbul but it's close to my home so I'm sure I'd enjoy it. It would definitely be exotic. Anyway, I'd recommend Rome because it's one of a kind. End of may, the weather should be wonderful and there are so many things to see. I was there in december and kept thinking I may have made the wrong choice getting stuck in Milan!
Whatever you choose, I have 1 advice: HOLD ON TO YOUR PURSE, ANYTIME, ANYWHERE!