It's relatively affordable and the most stain-resistant among the manmade varieties of carpet. Wool is used occasionally, but it's very expensive and, ironically, not as durable as nylon. Homeowners select wool primarily because of its luxurious feel. It's not particularly stain-resistant, however. Some manufacturers also make carpeting of polyester, which is comparatively softer than nylon but less durable.
Polyester provides a nice feel underneath the feet and also resists stains and soil quite well.
It's also less expensive than nylon. You'll also want to purchase a high-quality padding. Many homeowners skimp here because they think it's less important than what's lying on top, but it's not. The quality of the padding you select will ultimately determine how much mileage you get out of your carpeting. It's also what determines the absorption of heat, cold and sound in your home.