My short answer is that it's so close I don't think about it one way or the other.
If I'm going solo then my hammock is lighter. If I'm going with someone else and sharing a tent then no, it's not always lighter to use a hammock. But on the other if I'm in a tent I take a heavier more comfortable pad so that adds weight to the equation.
Even in the instances where using a tent would be lighter carrying a extra few ounces is worth the added comfort to take a hammock.