There are 3 basic differences between the flour and the cornstarch as thickener. By appearance, flour creates an opaque gravy while with cornstarch, it creates a clear and shiny sauce. By flavor, cornstarch doesn't have a roasty-toasty flavor like flour. You need to cook flour enough for it to loose the raw flavor. So by cooking time, cornstarch takes a short time to thicken. Plus if you cook cornstarch too long, it thins out again.