The similarity, in my experience, is more visible when held in relation to the East Coast. For west coast cities, you're probably right, but I'd bet most people would consider Seattle closer to SF than to NYC or Boston. Someone from the east coast would never call NYC and Boston similar, but they're closer to each other than they are to SF or Seattle.
Having lived in and around both, I would say NYC and Boston are fairly similar. Boston's whole psyche is based around staunchly insisting on not being inferior to New York.
