Getting down to the nitty gritty...

Flooring Fibers... The Good the bad, the do's and do not's.
This is my opinion.  Some people share it and some people may not.  My Mother and Father always tell me what is my opinion to anyone else (sarcasm included).
I am not going into ALL of the fibers just a few main ones that are more popular today: Nylon and Polyester

There are pros and cons about ALL flooring.  Make sure that you are looking at the flooring that best fits your needs.

A few things to know as you start your shopping...

1.Carpet goes down lighter like paint goes on darker.
2.The tighter the twist the better the wear.  If you have a blown out yarn that may feel thick, is the twist isn't tight that carpet is not going to wear well.  That creates a problem.
3.Everyone knows this but they hate saying it... You get what you pay for.  If your carpet is cheap or as we could better call it 'inexpensive' or 'builder grade' it is not going to look good for 20 years. Yes, you can get a great deal or purchase carpet that is on SALE that will be less expensive and wear a bit longer.  But you cant blame your carpet for not wearing well if you are not maintaining it. Carpet should be professionally steam cleaned once a year.  You also need to vacuum your carpet once a week per person that lives at your home, and animals count (eg. 4 people and 2 dogs and 1 cat- you should be vacuuming every day)
4. Don't just buy your carpet for the warranty.  Warranty have a lot of fine print so look at the texture retention (how long the carpet should look good) warranty.  Look at the wear warranty. The information is good.  But if you get your carpet based only on warranty make sure that you get the warranty book so you do everything correctly. The place that you buy your carpet will be able to provide it or you can call into the manufacturer and get it also.
5.  Soft carpet is very popular. I love it also, I love to pet it as I sell carpet.  Something to be aware of is that some vacuums have a hard time with the soft carpet so that is something to consider when you decide what carpet you want to install.
6.  Bare feet people... I am sorry to tell you this but you are ruining your carpet. SOCKS are what you should have on your carpet. Shoes would be a close second (as long as they are not covered in mud, tar, dirt, etc.).  The oils in your feet cause the dirt to stick you your carpet.  Giving you (what I call) dirt trails showing you all the places that you walk while you are bare footed.  A good example is with flip flops, you wear them all day and when you get home you take off your shoes and your black feet are so graciously cleaned by the carpet.

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As you read on you have to remember that this is my opinion...

Nylon-

Nylon has been out for a long time and has gotten the well wearing name.  Also most of the carpet are treated with some type of stain protection they also clean well.  But If you are looking for carpet for your hallway and stairs I would make nylon my top recommendation.

Polyester-

Polyester came out a while ago, it has a good clean ability but it does need to be maintained a little bit more.  

Solution Dyed.... any fiber Polyester or Nylon is great.  Solution dyed fiber is the greatest stuff.  If you spill bleach on your carpet, it will maintain the color.  This I do not recommend doing because it can eat your backing because bleach is such a strong chemical.  Solution dyed carpet gives you carpet that is easily cleaned and very hard to stain.  So if you get a solution dyes nylon or polyester you know that it will clean sooo well.

I do not want you to be discouraged though, most carpet has a stain protection on it.  This helps you get different spills out of your carpet. Something to think about still is that when you spill something it can go into your carpet padding.  When you clean the spill out of the carpet there is still some in the pad.  So when you have that phantom reoccurring stains... it could be because you have some of the mystery substance in the pad.

So in my opinion get a fiber that will best fit your needs.  If you have a high traffic house you might want to consider getting a nylon carpet.  If you have hard surface in all your high traffic areas then you get whatever you would like.  Everything with your flooring is up to you because you are the one that has to live with it everyday. Make sure that you also are looking at the quality of the carpet.  If it has a blown out yarn, this will only get worse over time.  The tighter the twist the better the wear.  Make sure that you are doing your homework.  There are so many different fibers that you can use when getting new carpet. These are just the main ones that I like to sell.

This blog has a really good insight if you wanted to check it out for another opinion.
http://puchers.com/blog/2011/07/29/nylon-vs-polyester-carpet-which-is-better/