Neither is particularly well-suited to the centro storico. W/that said - the Metropole will have better public transportation options and is closer to the train station. The Novili is in a slightly more picturesque neighborhood.

However, staying at either for the purposes of visiting Florence is like staying at EWR Hilton or JFK HGI when visiting Manhattan. It can be done, but you aren't staying in Manhattan, likewise for the Florence options.