The burn rates are very close... look at IMR's burn rate chart. They are both billed as versatile powders with a wide range of applications. Of course specific load data should be consulted for specific loads.
Point taken but I still don't think their interchangeable . I tend to follow published load data . I guess I'm just not very adventurous when it comes to reloading.
Luck is the phenomena created when Preparation meets Opportunity .