The only reason electronic loads are better vs power resistors, is that it is continuously adjustable (or in very tiny increments) and its transfer function can be changed from resistive to constant current, constant power or constant voltage.
Having said that, a 1200 watt e-load is not a trivial design. Not for a beginner. My first e-load was only 50 watts, and took a while to correctly design the closed loop response, otherwise it will fight against the UUT's own control loop.
If you are ONLY planning to test a single or very few supplies, nothing beats the cost and simplicity of a bank of power resistors. With simple switches you can make it adjustable (in coarse steps).
Otherwise, it is best if you purchase or lease a commercial e-load as Easy Peasy has suggested.