Depends on how you define 'impact' or 'success'. They both had a lot of impact back then. Britney always was more popular and she was very big in the 2000's because of the catchy music, being pretty, being cute/**** and great dancing. Xtina was popular and respected for her voice, had some catchy songs as well and more serious music. She was unlikeable as a person, so that's why she was less popular imo.
Since 2005/2006 Britney is known for her private problems and she has became throwback thing. She's being praised for what she did 20 years ago, while Xtina has grown as an artist and is still being praised for her voice and being the voice of multiple generations. She's still a better singer than a lot of popstars who are popular now.
Of course Britney is more popular now still, but not for the right reasons. It's because of the c-ship, her weird insta posts and her music from the 2000s. She isnt respected as an artist, because she doesnt sing or dance.
Christina is free, is living her life happy and is still in inspiration for a lot of people as an artist. So I think Xtina is more succesfull in general and in life.
Sorry for my English