UA has connections via ORD, DEN, and IAD to many of the OPs destinations and UA has the most nonstops to California and Colorado, two of the OPs destinations. Add to that USAirways' network of flights along the east coast - especially NY and DC and Florida also mentioned by the OP - and I think you have the best coverage for any one program to the OPs listed destinations.

AA out of BOS is relatively weak to NY, DC, Ohio, and Georgia with no partners to fill in. DL has no nonstops from BOS to either California or Colorado.

As a secondary point, the Red Carpet Club in Boston is one of the nicest in the country and directly across from UAs primary gates.