For large quantities: resistive - because almost all the conversion from electrical energy to heat can be arranged to occur within the water. The heating of other components (the water container) can be minimised - I expect the efficiency is probably inversely proportional the mass to a power about 1.

For Small quantities microwave is probably better - here I'm assuming it can heat up a small quantity much faster than the other components (the transmitter is probably the greatest source of losses) - I expect the inverse proportionality of efficiency to mass has a power significantly less than 1.

Where's the cut-off - I reckon about 0.1kg.