Lady Gaga, because she's talented and actually influenced Katy herself.
She's also basically the figurehead of the digital age and its pop stars. Katy's cute and all but... no.

Katy

Gaga. Since she's already influencing her peers. Katy, Rihanna, Justin, Nicki and even Beyonce. All of these women are already taking notes from Gaga.

Gaga, of course. Regardless of what you think of her, you have to admit that she's a visionary. All Katy does is make generic pop songs with no soul, and no personality. They are catered to the non-thinking, young crowd who just want songs to dance to. She may be big right now, but when the day comes when she isn't, that'll be the day that everyone forgets about her, because her songs are catchy, but that's it. Her songs have zero substance.

I don't want to insult both, but do we miss them when they are not on a campaign?

I love ADELE, but hey there is so much more music around.

And musically, there is all said and done. I'm almost certain that there are not much on new music styles to evolve.

It's now the time of the "eternal recurrance of the same".

Katy inspired Gaga to release an inspirational song (she was gonna release Mary the Night, but after seeing the success of Firework she copied).

Katy inspired Gaga to dye her hair blue (she did teal though, which is as close as she can get without being obvious).

Katy inspired Gaga to dress "cute" after Marry the Night was a disaster on the charts (trying to be like the most successful artist out there = Katy).

History repeats itself.

Gaga. Without the release of TF, TD would never be possible considering TF was a slight inspiration for making that album.

For cooky outfits : GaGa

In terms of massive eras and single success: Katy

GaGa. i dont like little monsters, but Gaga is more talented (compose, sing...)

Gaga by miles

Gaga

It's obvious ..

Gaga. She's a legend. Katy is already forgotten

Gaga. Especially worldwide.

The one who's the female MJ.

Receipts?