Synthetic haired brushes are usually recommended for liquids and creams and natural haired brushes are usually recommended for powders though I personally don't stick to that rule. I generally prefer natural haired brushes because of the reduced dry time, general softness, and I tend to work more with powders than I do creams and liquids.
Photos courtesy of Real Techniques and Nordstrom
If you had to pick, which would you choose? Synthetic or Natural Hair Brushes?