Bronko hit it right on the head there. Its just one is smaller in grain size so you get a little more into each measure of powder. You get more velocity out of the 3f and depending on the brand of powder it can be a great deal. Triple Seven for instance, the difference is not all that great. But I do like the 3f for getting large chunks of lead moving or cold weather shooting. Swiss on the other hand, the difference is almost 15% in some cases..

The best thing is try try each of the different kinds of powders and see which of them gives you the best accuracy. Some of my rifles like 3f. Other hate the stuff. All you can do is try.