Sorry, but it's definitely Gaga's... Hers are actually interesting. They are striking and avant garde.
All I can say about Brit's is that her covers look "pretty". She looks hot as hell, but it's not anything original and it didn't leave a lasting impression on me.
The "V" stands for Visionary. As much as Britney's cover slays, it's too simple, her whole shoot could have been just a perfume ad. Gaga fits the purpose/audience of the magazine a lot more and the cover is a lot more original than Britney's.