First, think fiber.

1. Nylon carpeting is durable, stain resistant and repels mildew. It holds color well, though prone to staining if not given a stain-blocking treatment. The vast majority of residential carpeting is nylon.
2. Olefin (or polypropylene) lacks the soft feel of nylon that homeowners love, but is strong, stain and mildew resistant and inexpensive. Olefin is a popular choice for commercial applications.
3. Polyester carpeting can be actually softer than similarly priced nylon. Sometimes made from recycled plastic bottles, the cost and green aspects are great. The fiber is prone to crushing, though, so there are wear concerns.
4. PET or PTT carpets are a sub-class of polyester and gaining tremendous market share. They can be as durable as nylon, but softer, better stain resistance and lower cost. Over the last several years,  the multi-family residential market is moving more towards PET or PTT carpeting.
5. Wool carpets are both luxurious and naturally stain resistant, though the very high price eliminates wool from most discussions.

Within a type of fiber, it's fairly easy to compare one carpet option to another. Simply put, the more dense the carpet (in oz per sy) and the higher the twist level (in turns per inch), the more durable. After you understand the fiber, think about the style. Carpet is made with the fiber looping into the backing. Loop carpets retain those loops and cut piles slice them.

1. Texture carpet is a cut pile.
2. Frieze carpet is a cut pile with a tight twist in the fiber.
3. Berber carpet is a tight loop that offers great durability, usually without patterns, named from the Berber people of North Africa who made carpets with knots that resemble the tight loops. Berbers are mostly used in commercial, high traffic areas.
4. Loop carpet tends to be a lower cost option with less aesthetic options.
5. Pattern carpet can have fiber depth patterns or fiber color patterns.

No rational method currently exists to compare the various fibers and styles. As stated earlier, comparing within a fiber group can be done based on density and twist level. But there is no good method to compare 24 oz/sy nylon carpet to a 26 oz/sy PET. So the comparing of carpet options, particularly when approving "Or Equal" products remains subjective and challenging.

The carpet padding, used in most residential applications for a more cushioned feel, should be at least 6 lbs/cf. Most commercial carpet installations are glued directly to the concrete floor.

If you are trying to value engineer a building project, understand that substantial savings can be made by approaching the carpet selection from an informed viewpoint. It's definitely not a situation where "The more you pay, the better you get."