Coming from a person who is trying to maintain your carpets appearance for a living, here are some considerations when buying new carpets.
First, Do you have pets or small children and are you able to keep up on the housework. For example, do you or your housekeeper vacuum regularly. Also, do you respond to accidents on your carpets promptly? If the answer is yes to both questions, you should consider buying higher grades of carpets (more fiber content per square foot). With proper care, the higher grades of carpets should last longer and provide a better return on your investment. If the answers above were no, a higher grade carpet will probably not last very long and you might want to look at lesser grades of carpets.
Sculpted carpets (carpets with high low patterns) are quite attractive but should be used in rooms that are not high use rooms. These types of carpets will show wear patterns quicker due to the high low nature of the fibers. The high spots do not have the support of neighboring fibers and will get worn down to the level of the low fibers resulting in a visible wear pattern.
Finally, Berber carpets are also quite attractive but once again should be considered for rooms that are not high useage. A Berber carpet, by the nature of its weave, is hard to clean when dirt gets into its fibers. Also, they can unravel if the loops get damaged (cats or dogs scratching can create damaged loops) and the less expensive Bebers tend to show wear patterns quicker.