I am surprised nobody has mentioned IBERIA. It may not be the best airline that's out there (in terms of service, etc), but it would work a treat for this itinerary (BOS-MAD-BCN // VCE-MAD-BOS).
Gives you:
- 1 easy change at MAD's new Terminal 4 both ways
- A340s on the transatlantic flights with the new BusinessPlus seats
- A320s on the European legs (nothing special in terms of seats, but that's the same as any European airline)