I'd say Selena got more global exposure through Disney Channel than Jessie has received from any of her singles thus far, even though Price Tag and Domino were internationally successful.
Selena is the biggest Disney star at the moment and is definitely bigger than Jessie J Worldwide
Jessie J had the more successful career musically but when it comes to being famous or bigger Worldwide as how you want it to be said, Selena is the obvious winner
Obviously Jessie J. I don't think many of your guys realize how massive she is in the UK. Her fans in the UK alone are larger than Selena's twenty member ATRL fan group.