I stay at both the Commander and Westin Copley pretty regularly, and have little to add to the others here, except for one thing: atmosphere.

The Westin Copley likes to think of itself as trendy, a faux W (at lobby level) in a city without a W. Service, as a result, is cool and not especially friendly. It also is a convention hotel. But the rooms are good -- especially if you get upgraded to a corner room (aka "junior suite" in Westin Copley parlance) as a Plat. Expect a modern hotel without a lot of warmth.

The Sheraton Commander is historic. Many rooms have recently been renovated. If you can get an Exec King on a lower floor (which another FTer recommended to me), I highly recommend it -- a four-poster bed in a good-sized room. It is a historic (not fancy, but historic) hotel in a fascinating area.

To the points of others on being "central," the Westin Copley definitely is more central. But odds are you'll be taking the T in either case. The Westin Copley is closer to the T -- one block from the main entrance to the green line. (The Commander is on the red line, about three blocks away.) I'd recommend, in either case, buying a seven-day Charlie Ticket for $15, which allows unlimited travel on the T for up to a week. Considering the one-day unlimited Charlie Card is $9, it's a bargain.