There are different factors that will determine how a carpet will clean up, such as the carpets fiber composition, soils in the carpet, cleaning habits of the carpet owner, which methods were used for professional cleaning in the past, how old the carpet is, and so on. Both the carpet and the quality of the carpet pad, can play a role in how your carpet wears over time.
Polypropylene Olefin is often used to make loop pile carpet and holds up very well in high traffic areas, and cleans fairly well. Berber is not the best choice if you have pets as the loops can get snagged by a cats/dogs nail and in turn run the entire length of the carpet. It is also famous for for pet stains to wick back (coming to the top) after a cleaning.
Wool is a very durable and an easy to clean fiber but is not very common due to the cost. Wool can also be damaged if not professionally cleaned properly. Possibly a good choice for those that may not have children or animals present. Be aware to when there is humidity present, the wool carpet can emit a "wet dog" like smell.
As of now nylon is the top fiber found in the carpet, though it may not be for much longer. Nylons pile cut (composition) is found with a wide range of quality and therefor price, with cheaper grades around $5 a yard up to or more of $50 per yard for a denser stain protected version. While the cheaper version does clean up well, the wear over time of traffic patterns and stains will show more than that of a higher grade nylon. The builder grade is what is commonly rental units as the higher grade nylon fiber doesn't make sense; builders will also use it in new houses because it is cheaper and will look good for the first year for the warranty period of the carpet. If you are a homeowner installing carpet into your home and would like a carpet that will last a longer time, choosing a higher grade nylon carpet (most common Dupont Stainmaster) as well as a higher quality carpet padding will be the best choice. Overall nylon, is a favorite carpet fiber to clean because i cleans up nicely, looks good most places, and there is a more range in how the carpet can be cleaned (*steam cleaning/hot water extraction is recommended however).
This could be the worst carpet fiber that is seen when it comes to wear and cleaning ability, unfortunately this is the fastest growing fiber being produced and possibly take over both nylon and olefin. The quality of carpet fiber has been taking a turn for the worse and going down, and since polyesters cost is lower, it is being manufactured more and more world wide. Being a carpet cleaner, this is not a good thing as the wear and traffic patterns are more apparent and darker (shaded) in a shortened period of time as well as not cleaning up as nice.