Madonna came first, and Britney came second, huge records, huge impact, huge careers. It's very hard to compare them, because they peaked at different times in pop music.
She didn't make it, but like you said she made it comeback in a big way. Max Martin and Britney changed the way pop music is since the release I she first few albums.
Everyone wanted a pop smash, everyone wanted to be her, everyone made better videos! She brought it back big and changed it!
This is a controversial opinion around here, but I think that time will put Britney's career into perspective... I really think she's messed up her legacy big time over the past few years. She's not going to be as remembered as I once hoped.
Unless she has a huge comeback or return to form, obviously.