When an antenna datasheet states that it has 50% efficiency, I understand that to mean only 1/2 of my transmit power will actually be radiated. Does it meant the same thing when using that antenna to receive? Will it receive only 50% of the signal that reaches it as opposed to a 90% antenna? Or is antenna efficiency only about transmit power and there is another metric I should consider for the receive side of things?
Yes. Antennas are reciprocal devices, so receive and transmit performance should be the same. With a 50% efficiency antenna, half of the power is radiated and half of it is dissipated as heat. For any incoming energy, half of it will be coupled into the feed line and half of it will also be dissipated as heat.