What is the best way to get that carpet of green lush grass that everyone loves? The choice you make between sod or seed depends upon a few factors. The first is usually the price. Seed is definitely the less expensive option. With sod you are paying for the labor of the sod farm to raise and care for the grass as it grows into that lush carpet that provides you with instant gratification.
Which leads us to our second factor, your time frame.  If you are in need of that instant gratification you will certainly want to choose the sod over the seed.  Even assuming you are planting in the prime planting seasons (late spring or early fall) grass still takes several weeks to grow and it will NOT look like a lush green carpet right away. There WILL be weeds and it may even take a few seasons and lots of fertilizer to achieve the lush carpet echelon.

Third factor to consider is typically the site conditions; seed gives you more flexibility in the sun vs. shade varieties, or a mixture, while sod is normally the sun-loving strain. If your lawn is not flat and has steep slopes you might also want to spend the extra money for sod since the seed will very often get washed away in the rain.

Keep in mind that when you ask your contractor to install seed instead of sod that you are assuming the risk of the final outcome.  Unless the contract (and price) specifically states otherwise, YOU are now the caretaker of the new peach fuzz grass sprouting in your lawn and you are NOT allowed to withhold payment for laying the seed until such time as your peach fuzz resembles a lush green carpet – it is a simple risk/reward situation.