Brockton is about 30 minutes from Boston. It's not really a suburb -- it's an older city that used to have lots of textile and shoe manufacturing, but that has lost a great deal of lustre (not that it had a whole lot to begin with) in the last half-century. You would not want to take public transportation there.

The Westin Copley would be an excellent choice: you're in the middle of the Boston shopping area, and there are plenty of restaurants and places to explore nearby. It is not a difficult drive from the Westin to Brockton: you can get on the Mass Pike westbound (I-90) within a block of the Westin, and then it is all highway until you get to Brockton.

The Sheraton Braintree is not in a very good location at all; you need to drive anywhere you'd want to go. It is about 10-15 minutes closer to Brockton than the Westin Copley.