So, if it were my house, I would install NYLON carpets in the high traffic areas where food stains aren't much of a concern, like hallways.
In dining rooms, children's rooms or anywhere where food or liquid spills are likely to be more frequent, including anywhere where pets may be kept, I'd install polypropylene (also called "Olefin") carpet for it's excellent cleanability. Also, look for a "commercial" polypropylene carpet. These kinds of carpet will be made for longer wear.
The kind of pile you want:
Basically, the most durable kind of pile is a level loop. That where the carpet yarn simply forms a loop. The loops have a natural resilience to them, so level loop carpets are the longest wearing.
If you cut the loop at the top so that all the fibers in the yarn can move independantly, you have what is known as a "plush" carpet. Plush carpets will show an apparant change of color depending on which way you brush your hand over the pile. (Level loop carpets won't do that.) Some people like that effect, some people don't.
A "saxony" is a plush carpet where the yarns have a tighter twist to them, and there are more yarns per square inch. Basically, a saxony is a high quality plush carpet.
If it were my house, I would use level loop carpets everywhere except those rooms where you're most likely going to be spending most of the time in stocking feet, such as bedrooms. The plush carpets offer a softer, warmer carpet to walk on in stocking feet, and bedrooms don't see nearly the amount of foot traffic that dining rooms, living rooms and hallways do.
However, a plush carpet is difficult for people who are confined to wheel chairs or walk with walkers to deal with, so I'd go with plush carpets in your bedrooms and level loop carpets everywhere else. A berber is considered a level loop, even though the loops are so much bigger.
UNDERPAD:
Basically, the stiffer your underpad, the better it absorbs the impacts from foot traffic over the carpet. A better underpad will extend the life of the carpet, but no underpad is going to double the useful life of the carpet.
Underpad will be quoted in "pounds", and an "8 pound pad" is one that would weigh 8 pounds per CUBIC FOOT. Generally, you get the most "bang for your buck" with the 7 or 8 pound foam chip pads.
If you have a pet that makes messes on the floor, your best bet would be to buy a "pet pad". A pet pad is made from foam rubber where they reduce the amount of blowing gas when making the foam so that the bubbles inside the foam are NOT interconnected. Consequently, these foam rubber pads are not permeable to moisture, and any accident that a pet makes on the carpet will not seep through the pad and penetrate into the wood underlayment under the pad.
So, believe it or not, the best carpet installation would have nylon level loop carpets in the high traffic areas, polypropylene level loop carpets where liquid spills are most likely and plush carpet (probably nylon or polyester) in the bedrooms where a warm soft feel underfoot is desireable and there'll be minimal traffic or liquid spillage. However, most people don't want their floors to look like a "patch quilt" with different kinds of carpet everywhere, so the carpet that's best for your home would be a compromise between choosing how many different kinds of carpet you want to have and where to have each kind. Often people will go with one kind of carpet everywhere, and won't really care that a different kind would be more suitable and appropriate in a particular room. Other times, people will have different kinds of carpets in each room. If it were my house, I would not have any problem with having different kinds of carpet in different KINDS of rooms, such a adult bedrooms and children's bedrooms. Children are much more likely to be spilling Kool-Aid or fruit juices on the carpeting in their bedrooms.
Just be forewarned that any plush carpets you have will show an apparant change in color depending on which way the vaccuum cleaner was moving when it went over that part of the carpet. Some people don't like that characteristic of plush carpets, others do.
Here's an excellent web site for learning about carpeting:
http://www.carpet-rug.com
The CRI (Carpet and Rug Institute) is an institution put in place to provide information and advice to the carpeting selling, cleaning and installation sectors of the economy by carpet manufacturers who collectively account for over 90 percent of the carpet produced in North America.