Figure out how much appearance is important to you with the material you choose. Generally, gravel is a better choice when appearance is tantamount.
Consider the importance of soil improvement. Organic mulches that will eventually decompose, such as wood chips or straw, will improve your soil, while gravel will not.
Look into gravel if you plan to use the landscaped area for walking or as a garden path.
Factor in the wind. If you frequently get high speed winds, gravel is your best choice. If you want organic mulch, choose a heavier wood.
Think about the color you want. Mulches are natural colors like browns or tan, while gravel tends to be gray yet is also available in other colors.
Consider how hot your climate is. Organic mulches can be a better choice for very hot, humid climates because gravel builds up heat.