It is generally recommended to use the low setting when possible to allow the food to simmer together longer for more flavor. It may also help large cuts of meat become more tender and keep chicken breast from drying out. That said, if you don't have the time for the low setting, go with high - both the cooker and most recipes are made for either. I personally can't tell much of a difference and most people I know base the decision on what their schedule allows and it's all good. Specifically for this recipe, I choose high for the shorter time because I like firmer carrots.