When used in the salvation passages, Lord especially emphasizes the deity of Christ. We ask that he trust a Person who lived 2,000 years ago, whom he can only know through the Bible, to forgive his sins. Salvation is free. Lordship is very costly. Faith is a gift bestowed by God upon unbelievers.
Discipleship is a commanded work of obedience for believers.
Both faith and discipleship are absolutely important, the one for salvation, and other for sanctification.