Gaga before she was "Gaga":
Interesting to note (as the Youtube commenters did) that this is the same chord progression she ultimately used in Poker Face.
The difference between the two is that Stephanie Germanotta is largely a product of her own musical skill, invented into the persona of Lady Gaga and popularized. Britney Spears came from celebrity on the Disney shows, and really didn't have much musical talent or training that aside.
I despise music snobbery, and as PERFORMERS I think both Britney Spears and Lady Gaga are excellent at their goal. I really enjoy pop/dance music. As MUSICIANS or VOCALISTS, I think Gaga clearly takes the show.