Westin is the nicer hotel (facilities, staff), Summerfield has more space. Just out of curiosity, have you compared the rates for the two for weeknights? (I haven't, but it wouldn't surprise me if the Westin were more $$ Mon thru Thurs nights.)
All that said, the hotel locations seem odd given the plan. Waltham is a hike from Ipswich, especially on a summer weekend when you will have a lot of company on Rt 128 heading toward the Cape (Cape Ann, of course).