Gaga. Britney is more similar to Janet. The only reason why Britney was compared to Madonna is because she came out as a solo female pop star and there wasn't any in such a long time and she created a huge impact.
Gaga and Madonna get compared a lot more because they're both italian-american blondes who were edgy with mixing sex and religion, hidden messages and tributes, and weren't the conventional beautiful America is accustomed too. They're both smart creative women who's music and style is consistently evolving, and opened a lot of doors for fellow female artists.
Britney is a southern girl who likes to dance around and have orgys in her music videos, and if you haven't noticed most of her music is either about seducing someone, partying, or how paparazzi like to stalk her.