Durability depends on three important factors:  The type of fiber, the yarn twist, and the pile density.  Choosing a carpet based on any "one" of these factors can be misleading,  It's important to select the right balance of all three. 

Notice that weight of the carpet is not directly mentioned above, weight certainly is a factor in calculating the density rating of a carpet, and therefore has some bearing on durability, but weight alone does not address other major durability factors such as, yarn twist, fiber type, and texture. Comparing carpet based on weight alone can be misleading.

Carpets of good durability can be found in all of the "construction types" listed below.  Therefore, a good starting point in carpet selection is simply determining the look you like, such as smooth saxony, textured cut pile, looped,  patterned, etc., then you can look at differing durability factors with-in the style you like.

Fiber:
Nylon continues to be the ideal carpet fiber.  It is the strongest, most resilient fiber used today and is now available in very soft forms.
Polyester fiber produces carpet that are also soft to the touch, but not as resilient as carpet of nylon.  Polyester is best suited for low traffic areas.
Olefin (also called polypropylene) is often used in indoor/outdoor carpets and in loop-pile carpets.  Olefin is more resistant to stains but has less resilience than nylon or polyester.
Wool is a natural fiber and presents a beautiful appearance.  However, wool may not be as resilient at man-made carpet fibers.

Yarn Twist:
Yarn twist, particularly in cut-pile carpet, is important to performance.  All pile yarns in cut-pile carpets have been heat set to help maintain their twist.  The tighter the twist of the individual yarns, the better the carpet will retain its appearance.

Pile Density:
Density is how tight or how closely tufted the yarns are to each other.  Dense carpets tend to be firm and less soft because the pile can not move as much as carpets of lesser density.  The density "equation" does not address yarn twist, fiber type or texture, which are other important factors in selecting carpet.

Color & Texture:
Light-colored carpets will show soil more than dark colors, and dark colors will show lint more than light colors.  The appropriate color carpet can help disguise soil tracked in from the outside since the colors of dirt, sand and clay vary widely. Tweeds, pebbled textures, and patterns are good choices for rooms where spills and soiling are likely to be a problem.