Milltown. They handled all the paperwork for the rebate as well. All in all I was happy with the work - they had some issues with the thermostats and controls but they kept on it until they (and I) were sure it worked correctly. You also get a year of free service contract as part of the install.

Note that you'll need to get the MassSave efficiency assessment done to get the rebate, and also might need to upgrade your insulation.