There are many ways to thicken a sauce, from cooking it down slowly to adding one of the many available thickeners. Cornstarch is more commonly used than rice flour, but the process of using rice flour is simpler and more forgiving. Although there are notable differences in technique and behavior between cornstarch and rice flour, you really can substitute one for the other in almost any recipe. The results may not be exactly the same, but either ingredient will get you the thickness you need. Rice flour is more expensive than cornstarch, but you need so little of either thickener to get results that the cost difference is unlikely to break your budget.
Using Different Thickeners
To thicken a sauce with cornstarch, make a slurry by mixing 1 tablespoon of cornstarch with 1 tablespoon of cold water for each cup of liquid you wish to thicken. The extra water that you add to the cornstarch slurry won't increase the amount of liquid you need to thicken because it's such a small amount, and it will bind with the cornstarch even before you add the slurry to the dish. Stir the cornstarch mixture into your simmering sauce and then gently bring it back to a simmer. Cook for a few minutes longer, until you can no longer taste the starchiness. Don't cook any longer than necessary, though, because the cornstarch may break down and lose its thickening quality.
To thicken a sauce with rice flour, simply sprinkle it into the liquid you wish to thicken; then stir. You can use white rice flour, brown rice flour or sweet glutinous rice flour, which, despite its name, is gluten free. Use about 2 tablespoons per cup of liquid that needs to be thickened. Rice flour will thicken sauces almost immediately, so if your sauce doesn't feel thick enough after adding this amount, you can simply add a little more. To avoid clumps, be sure to sprinkle rather than pour it into your sauce.
Seeing and Tasting the Difference
Sauces thickened with cornstarch tend to develop a sheen. This quality can be appealing in sweet recipes such as puddings, but it can be somewhat strange in savory dishes such as gravies. Cornstarch also has a distinctive starchy flavor, but you can temper this by cooking the thickened liquid until the unwanted flavor disappears. Brown rice flour can be somewhat gritty, especially if you use a lot of it to thicken a large amount of liquid. White rice flour and sweet glutinous rice flour will give you a smoother result.
Choosing for Health
Both rice flour and cornstarch consist primarily of carbohydrates, but you don't use much of them and they can help you avoid unhealthier options for thickening sauces, such as butter and cream. Brown rice flour contains micronutrients not available in white rice flour or cornstarch, but, again, you really don't use enough to rely on it as a significant source of nutrition. Although cornstarch is made from corn rather than wheat, some brands are manufactured in facilities that also process wheat flour. If you're avoiding gluten, look for a brand that specifically states that it's gluten free. If you avoid genetically modified food, also check your cornstarch packaging to make sure it is non-GMO certified.