Hmm... I'd say Porunga is stronger then Shenron simply because Guru was "whole" when he created Porunga, where as Kami had split into two, himself and Piccolo, and was weaker when he created Shenron. That scenario only makes sense cause the Black Star DragonBalls were created when Kami was "whole" and they were much stronger then the regular DragonBalls.
