Selection of Vout_DC in 900MHz wireless Energy Harvester

I want to deliver Pload=3mWatts by 900MHz wireless energy harvester.

For example, I can assume following two.
(1) Vout_DC=1.5Volts, Iload=2mA, Pload=3mWatts
(2) Vout_DC=3.0Volts, Iload=1mA, Pload=3mWatts

Which is advantageous or effective ?

You'll probably use diodes in your receiving circuit to convert AC to DC. Incoming voltage needs to rise high enough to turn on the diodes. Suppose you can generate 2.9V max, then case 1 has the advantage. Otherwise it is easier to design for case 2.

Another approach to consider is impedance matching. For case 1 this calculates as 750 ohms (1.5 / .002).
In the second case 3k (3 / .001).
Therefore if you can obtain sufficiently low effective impedance in your receiving circuit, then case 1 appears to have the advantage. Otherwise it is easier to design for case 2.