Both sang on Ellen. Same venue. Same stage. Same speakers.

Yet... Ke$ha... sucked... Lady Gaga... showed talent...

Lady Gaga: YouTube - Lady GaGa - Poker Face (Live on Ellen) HD with Interview
Skip to around 10. that's when it goes to the version most people know.

Ke$ha: YouTube - Ke$ha - TiK ToK (Live At Ellen Show 01/05/2010)
The entire song... I could speak better than she just "sang"