Shenron is the dragon of Earth. Shenron only gives 1 wish per 7 dragonballs collected, but once Dende is the earth's guardian in the Cell Saga, Shenron gives 2 wishes. Once you've wished for something it can never be made again with Shenron.
Porunga is the dragon of Namek. Porunga gives 3 wishes per 7 dragonball's because the dragonballs are bigger on Namek than Earth so they store more energy. Porunga's wishes can be made over and over.
What is your favorite Dragonball Series?