The art of building a good floor register comes from physics. Because of the added control and the location, wall and floor registers are subjected to a constant change in air pressure, temperature, and sometimes even physical force. This means that vent cover must be built with care and an accent on quality and longevity.
The materials must be carefully selected for their ability to withstand temperature variations, physical force and be air resistant at the same time. One cannot just take a piece of steel and place it to cover the vent opening. That steel will warp, twist, rust, and eventually fall out of the air duct forcing you to rapidly replace it.
Solid wood is the material for looks and quality. No metal or plastic can give you the cozy feeling of a wood register. Oak, maple, and cherry are main wood materials used by people. Some others also include bamboo, beech, and birch. Solid wood also has an added versatility due to its staining properties. By staining your wooden floor register you can give it absolutely any look you want.
Plastic is the least expensive material of them all.
Solid Brass is the sturdiest of them all. It allows for amazing durability and air resistance while keeping the cost of the material relatively low. Brass is also known as a rich looking metal and gives the room that it is in an appearance of quality and high value.
Solid Aluminum is another metal used to make floor registers and cold air returns. It is not as durable as Solid Brass and is not as rich looking but it still does the job. Easily paintable it has the ability to be put almost into any atmosphere and thus possesses a touch of versatility in its use as a vent cover.
Steel is used only after being specially prepared to withstand the temperature variation that is required in the place of the floor register. Although physically durable metal, it is very much effected by natural forces that are abundant for any vent cover and thus requires special preparation. Steel is a good material due to its easily paintable properties as well. So even though it is not as good looking as Brass and not as versatile as aluminum, it is relatively cheaper and is a popular choice for a floor register built.
Plastic is specially designed to withstand temperature changes and thus is a perfect choice for anyone with a smaller budget. The only drawback of the plastic floor register is its looks. The technology has simply not come this far to mimic real material with plastic well. For any other practical purposes it is a fantastic supplement.