I've found that bull bugling is primarily good for locating bulls at a distance. Once in close, a cow call seems to work best. However, this is not always true. Different bulls react differently to close-in bugling and grunting. Some get pretty crazy and respond agressively to bull bugling and grunting. Others may answer but won't come near. Others respond by sneaking in without a sound. There is no hard fast rule of thumb, but overall cow calling works better because it is an attractant rather than a potential threat to the bulls.