They both have the same result but there are cases where each is more suitable:
For normal SAR data wich good orbit information and geocoding, Range Doppler Terrain Correction works the fastest and gives good results.
If your data lacks of good geocoding or you experience shifts in your data, you can apply SAR Simulation Terrain Correction which uses the variation in backscatter intensity caused by topography in your image to fit it to the earth’s surface by simulating the terrain effects based on the incidence angle.
Have a look at Figure 2c (https://www.gamma-rs.ch/uploads/media/2013-2_PixelArea.pdf) to see how this works. You however need pronounced landforms and a DEM of suitable quality. Otherwise there is no pattern to match in your image.