They are both very talented, but I would say Lady Gaga. She has way more artistic range than Beyonce. Beyonce's music is kind of repetitive and sounds the same.
Bey have stronger voice and can dance very well, but that's all.
And Gaga is a song writer, she create ALL her music (except instrumentals obviously) she can play instruments, good voice + good dancer(when she wants) + innovation. Gaga creates; and that is the definition of ART(IST).