Madonna. Gaga will do very well but Madonna been in the industry 30 years while Gaga has only been here for 3 and a half. I hope to see both. I'm sure they'll both deliver amazing performances
It just comes down to who's tour is longer and how big the venues are. But it's obvious Gaga's not going to outsell Madonna, especially since it's only her 3rd tour. Both will be very successful nonetheless