My absolute favorite hotel in the combined chain is the Gritti Palace in Venice. A screaming deal at 60k points per night, book quick. Normally I'd never transfer UR into Marriott Reward points, but if you can take advantage of the 60k points per night that will soon go to 90 points, it's like getting a 50% bonus (still doesn't get a Marriot point to an UR point, but it's Venice). Nothing like sitting on the Grand canal for breakfast, free for titanium. But it's $$$$$, I save my points for there.
I've been at the Hilton in Rome, and decided to stay there in September; the rest of the hotels are legacy Starwood, which is really strong in Italy. Westin Florence and one of the propoerties in Sardinia. I haven't been at the Hilton's overnight, but my rakings would be high end Starwoods at the top. The Cavalieri in Rome is stunning, the Excelsior in Rome is well located. I'd suspect the latter is better on upgrades but don't have recent experience.
I think upgrades in Italy are more likely at Westins than St. Regis because the St. Regis can get a member of the Saudi Royal family booking an entire floor. I'm a bigger fish if the pond is smaller.