Minimally, both groups practice baptism simply because Jesus commanded it. I personally don't think it's necessary to have any theories on what it accomplishes beyond that. I'm not sure you need to choose just one. Baptism is God's gift of grace and also God's summons to respond to that grace. God's faithfulness signified in Baptism is constant and sure, even when human faithfulness is not.
Baptism is received only once.
The efficacy of Baptism is not tied to the moment when it is administered, for Baptism signifies the beginning of life in Christ, not its completion. God's grace is working steadily, calling to repentance and newness of life. God's faithfulnss needs no renewal. Human faithfulness to God needs repeated renewal. Baptism calls for decision at every subsequent stage of life's way, both for those who Baptism attends their profession of faith and for those who are nutured from childhood within the family of faith.