Read the reviews under the Hilton board for the Hilton Garden Inn. It is a very easy bus ride to the train station or Duomo area. The hotel is like an American Hilton. I did a 7 night stay there last year and did side trips to Lucca, Ravenna and Bologna.
I can also recommend the Hotel Davazanti. It is located right off a street in the heart of the historic centre. I personally prefer this location over the Hilton Garden Inn but I saved a ton of money at the HGI last year.