Not a fan of either Christina or Ariana (I can't even name you any of Ariana's songs) but their impressions are pretty darn good.
Christina on Jimmy Fallon
https://www.youtube.com/watch?v=uD2nOjV3AaI
Ariana on Jimmy Fallon
https://www.youtube.com/watch?v=ss9ygQqqL2Q
Christina on Jimmy Fallon
https://www.youtube.com/watch?v=uD2nOjV3AaI
Ariana on Jimmy Fallon
https://www.youtube.com/watch?v=ss9ygQqqL2Q