Lots of info on the topic here, but you'll find that the St.Regis is perhaps a 5 minute further walk to Trevi than the Westin. The Westin is very close to Spanish Steps. I love the St.Regis, but have tried all the Rome hotels (except for Golf).
St.Regis: My personal fav in Rome. Love the rooms, service, location (near Brioni), just love it.
Westin: very nice, service was poor on my first visit, but after speaking with managment team, my second stay was outstanding. The hotel is larger than St.Regis, and not quite as impressive in terms of decor, but still most acceptable.
Sheraton: Further removed from the action, BUT does offer free airport AND downtown shuttle (similar to Hilton Cavalier), both take about 30 minutes depending on traffic. Fantastic lounge with very impressive spread, nice pool area and decent service. If you had some kids 7-10 I would recommend this hotel, as it would cater to them more than the other Starwoods.
4-PTs: Excellent value, drink coupons on check-in, nice rooms. Shuttle to airport and town, but further removed from area. I would recommend this hotel if you get in late on the first night or for the last night of your stay before flying out. Excellent price, great service, in new building.
Really just too far removed to enjoy central Rome though if the intent is to explore the old city.
email me if you would like more Rome info and be advised that priceline offers some good options for downtown Rome.