Depends on the particular artificial fiber used. Very generally speaking, acrylic knit beanies are the low-cost, mass-market version of a watch cap. It will be lighter and less warm than wool or synthetic fleece. it may have a looser knit pattern, and offer essentially no resistance to wind. But is should be cheaper to buy, easier to wash frequently, and quite durable. It will absorb and hold less water, and may dry quicker, but it is likely to be far less warm when it is wet than wool or some better synthetics.

It should be fine for the occasional, casual user just looking for a bit of additional warmth. For the outdoorsman, or anyone who really needs to stay warm to stay alive or keep working in foul, cold weather, invest in a better product.

Jeff