Is it better to seek forgiveness with Sayyid al-Istighfar due to its immense reward, or to frequently say "Astaghfirullah" (I seek forgiveness from God)?

Prophetic remembrances (dhikr) are recited in their designated places. Thus, seeking forgiveness after prayer is done by saying "Astaghfirullah" (I seek forgiveness from Allah) three times, and this is better than "Sayyid al-Istighfar" (the master of seeking forgiveness) in this particular context. As for "Sayyid al-Istighfar," it is recited among the morning and evening remembrances, and it is better at that time. Absolute seeking of forgiveness by frequently reciting the concise formulas such as "Rabbighfir li wa tub 'alayya innaka Anta at-Tawwab ar-Rahim" (My Lord, forgive me and accept my repentance, verily You are the Oft-Forgiving, the Most Merciful) or "Astaghfirullah al-'Azim alladhi la ilaha illa Huwa al-Hayy al-Qayyum wa atubu ilayh" (I seek forgiveness from Allah, the Magnificent, whom there is no god but He, the Ever-Living, the Sustainer of all existence, and I repent to Him) is better than reciting "Sayyid al-Istighfar" a lesser number of times.