Asking Allāh for steadfastness is greater then asking for his forgiveness, because a servant may be forgiven his past sins (as a result), but there's no guarantee for his future. Of course, both supplications are great, but what counts is how it all ends (hence the importance of asking for steadfastness in faith until the last breath).
Sins being forgiven is a blessing greater then all the good things of the world and it's ownership. Indeed, Sulaymān (‘Alayhis Salām) had the greatest kingdom, yet he asked forgiveness before asking for that kingship.
"He (Sulaymān) turned to Us and prayed: ‘Lord forgive me! Grant me such power as no one after me will have..." (Sūrah Ṣād (38): 35)
It's recommended to express repentance when asking Allāh for your needs; because it may be that a person has a sin preventing his supplication from being accepted.
"Then in the (veils of) darkness he cried out, 'There is no deity worthy of worship except You. Glory be to You! I have certainly done wrong.'" (Sūrah Anbiyā’ (21): 87)