TIM has probably the best WCDMA coverage in Italy, but its offers are also very expensive. I've never been there, but I suggest you to think about 3, because it has very good prepaid data plans (100MB/day with only 3euros/week).
On tre.it (three site) you'll find an useful interactive coverage map, which can be found here:
Trova Coperture, Negozi e Centri Assistenza 3
Otherwise, TIM has a really good data network, so you can use it without hesitation.
