I heard both of them live and i must go with Lady Gaga! No one can sing as good as her! Gaga has tremendous strength and at the same time has such a sweet voice. Her voice is so versatile, she can do pop, rock and jazz. Katy perry has a soulful voice but has a lot of faults when singing. She does not have the ability to sing as well as lady gaga. Lady Gaga is certainly, the voice of the generation and may even be the artist of the generation. Her new tour is EXTRAORDINARY!