Based on which of the two models (D415 and D435) has been shown to have the better depth image quality so far based on user feedback, I would recommend the D415. Whilst the D435 excels at capturing moving objects, there probably won't be much body movement involved in face recognition and so the D415 should do the task fine.
My recommendation would switch to the D435 if you were planning on recognizing more than one person in front of the camera, as the D435 has a wider field of view and so has less potential for blind-spots than the D415.
