I mean the entire, over the top thing, Christina has indeed done that before. She only does it once and a while, I honestly believe that she's doing it more now, since it seems to be the new thing, look at Nikki, look at Beyonce, lately being over the top seems to be the new thing.
But as a singer, Christina wins that hand down, I'm not saying that she wins lyrically, I have no clue if she does write her music, but she is a waaay talented singer, the octaves that she can reach is amazing. Yet, she never truly shows them off. If I remember she can reach 4 different Octaves, and that is EXTREMELY hard to do.
I like them Both because they have very beautiful voices and make really great songs. but lady gaga is more like madonna and you can't compare Christina Aguilera to anybody her voice is AMAZING and she's Beautiful.