Seeding
Seeding is well-suited for repairing or reinforcing an existing lawn. While it will certainly cost less, the maintenance required for the achieved result is, in many opinions, not as worthwhile.
You generally won't be able to fully enjoy your lawn until a few months after seeding. It's also important to remember that weeds are likely to appear quickly, as the grass won't immediately cover the entire area.
However, seeding can be a good solution for those who have time for maintenance and have a green thumb.