Is it best to fly to Bologna and take a train or bus to Renazzo?

Bologna is much more convenient. The airport is easy to get in and out of; it's the number 3 airport here in Italy in terms of traffic, so there are a lot of options. If you land in Rome, you would still have 3 to 4 hours on the train to get from the airport to Bologna (73 euro) to go to Renazzo anyway. There is no train station in Cento or Renazzo di Cento. It would be more convenient to rent a car.