Wool is one of the most commonly used materials for area rugs. Typically derived from sheep, wool can also be sourced from goats, or even alpacas. Silk is typically used either alone or in conjunction with other materials. The fiber is naturally derived from the cocoon of a caterpillar, and is often found in Oriental rugs.
In terms of functionality, it's hard to beat wool. Wool is extremely durable and fire-resistant, and does not stain easily. Because wool is hypoallergenic, it is a great material for those who struggle with allergy problems. Silk, on the other hand, is quite fragile in comparison to wool. Tending to stain and tear easily, silk should be reserved for situations in which it is not likely to be harmed.
Since wool is so common, its cost is fairly reasonable. Wool rugs are not typically viewed as luxury items, and are thus affordable for people of practically all budget levels. Silk, however, is one of the most expensive area rug materials on the market. Even when silk is added to a rug, it can raise the price tag quite a bit. Rugs that are made entirely out of silk are almost always extremely expensive.
Wool rugs are quite easy to care for. Because they are fairly resistant to stains, general upkeep is usually all that is necessary, with occasional vacuuming being the most laborious part of the maintenance process. Silk is not nearly as easy to clean as wool, as it is a far more delicate material. Hire a professional rug cleaning service to clean a silk area rug.