Which is more efficient over 300 to 500 miles high voltage AC or high voltage DC?
HVDC is more efficient over such distance.
Here are the reasons:

There is no inductive/capacitive reactance in the case of DC whereas in case of AC both capacitive/inductive reatances exist. Due to the absence of Inductance, the voltage drop in HVDC is very small as compared to AC (Provided that all other conditions are constant). Due to this reasons, the HVDC provides an edge over the voltage regulation of power system.

HVDC is free from dielectric losses. Also, there is no skin effect of conductor and whole conductors is utilised for power transmission.

In terms of cost of power system DC is more efficient and is least expensive. Firstly we only require two conductors instead of three. But the only problem we suffer is a generation of power at high voltages because we need HV for transmission. DC can neither be stepped up directly and neither can it be generated at very high voltages. So at present, the only solution is to use Solid state electronics and convert HVAC to HVDC for transmission purposes, then again convert it back to AC and step down for transmission purposes.