Which is better and more rewarding: to increase supererogatory (nafl) prayers or to increase prayers upon the Prophet (peace be upon him)?

If it is possible to combine abundant prayers upon the Prophet (peace and blessings be upon him) with abundant supererogatory prayers, that is best. As for preferring one over the other, it appears that abundant supererogatory prayers are better, because they are superior to engaging in recitation and include prayers upon the Prophet, recitation, remembrances (adhkar), supplication (du'a), and physical acts of worship.