jg70124,
I spent a couple of days in Belgrade (holidays) in 2002. I also like Paris very much, but I would definitely recommend Belgrade this time. As you said, you might not have another chance to visit the place again - so I think there's your answer.
Can't really say much about food - I recall we went to some cheaper places, and the food was nothing special. However, cafes, pubs and bars were great, and I also liked the atmosphere in the city. People were very polite and helpful - and the absence of tourists was a major plus. Taxis were very inexpensive so getting around wasn't a problem at all - and people tended to be very honest; waiters in restaurants didn't even accept tips!
BTW, I recommend you visit the aviation museum in Belgrade - it's just a few minutes' walk from the airport.
have a nice time in Belgrade!
okko