Whether you are replacing the grass in a bare area or trying to start an entirely new lawn, you will have to choose between sod and seed for the grass installation. There are benefits and drawbacks to both, but the following information will help you choose which option is best for your specific needs.
A square foot of sod costs more than seed for the same coverage area. With both options, you will need to remove old turf and weeds from the area as well as bring in fresh topsoil, compost, and fertilizer to improve soil quality. Since sod is already mature grass, you can often succeed with planting with only rudimentary soil preparation — turf removal, some light tilling, and maybe a bit of compost. Seed will require more intensive preparation, which comes at a greater cost, so that the grass has the nutrients it needs for quick growth and to ensure there is no weed competition. Yet even with the higher cost of soil preparation, seed is still typically less expensive than sod. If cost is your main deciding factor, seed is the better option.
It can be difficult to contrast seeding to sod installation on the merits of effort. Both options require the removal of the turf. The best way is to cut out the old turf layer in strips, roll it up, and dispose of or compost it. If the turf is patchy, you can simply till it in instead if you are planting sod, but tilling isn't recommended for seed since some roots may survive and regrow to compete with the grass seedlings.
Once the sod is laid or the grass seed is spread, though, seed becomes a lot more effort-intensive. With sod, all you need to do is water it once or twice a day and wait for the roots to establish. With seed, you must keep people and animals off the area, and then water several times a day to increase seed germination and prevent the seed from blowing away. You must also overseed later to thicken up bare areas where the seed didn't germinate, was washed away, or was consumed by birds. Overall, seed requires more effort to establish compared to sod.
If an instant lawn is your desire, then sod is the clear winner. A sod lawn can be used lightly within days of being installed, and it's typically fully established within a week or two.
Seed, on the other hand, can take a month or more before it is established enough to mow and walk on. It can take even longer before you have the full, lush lawn that sod supplies almost immediately.