The short but not really useful answer is stiffness, aerodynamics and mass.
Light wheels will allow better acceleration and climbing, because the rider has to put energy into the increasing angular momentum and lifting the mass against gravity.
Aerodynamic deeper section but heavier wheels are faster on the flat at a constant speed. Once the wheels are spun up mass does not need energy to keep spinning, and the aerodynamic losses predominate.
Generally a 'better' wheel will be one designed for the particular conditions you want to use them for, and there is a good correlation between quality and cost.
If you want to figure out which is the better wheel from similar designs from different manufacturers at the same price point, you'd need a testing lab to distinguish the differences.