I could not find any reference for comparison of these 2 models for the purpose of 2D hydrodynamic models for river.

On my personal observation, I think Delft3D must be better for river flow modelling since one can incorporate various other modules including tidal and waves in a better way as compared to HEC-RAS which has limited functionality.

However, there appears to be more references for HEC RAS 2D as compared to Delft3D which is strange to me.