Undoubtedly Lady Gaga. Her career will most likely have more longevity as well.

I can see Gaga being the Britney of this generation, and Katy is the Christina. Katy is at her Stripped days right now.

The one that is already a defined icon and is making her way to becoming a legend.

Lady Gaga

I see Gaga being remembered for sure. She's put herself so far OUT there, for better or worse, nobody'll forget her any time soon. Not just for her music, but her 'antics' (crazy fashion, over-the-top performances, even that "vessel" she arrived in) as well. Plus her outspoken role in the political and social field. Maybe her movieS too (we'll just have to wait and see about those!)
Hard to say the same about Katy. She's obviously very popular these days, breaking records here and there, but I dunno if she'll stay in my mind. When I think back on music nowadays, I'll think of Gaga, and probably Rihanna, before I think of Katy. Even with her movie that I never saw. Or her voice role in the Smurfs, which I also never saw.

Katy tbh

Gaga but not for her music.

Lady Gaga has already earned too much not to be more remembered than Katy..

At one point Lady Gaga was so popular WW .. a peak Katy will never ever reach .. I mean with 5 smash hits she couldn't even get close to reaching Gaga's peak.. And as we can see that Gaga is keep experimenting she may have her next big moment in Pop culture, while Katy working with Dr Luke may have again her big moment (having 4-5 number 1s).. you decide for yourself which will help more for someone to be remembered ..

Both.
Both will be little Icons in the end