If we look just the facial features, Gaga is way prettier. I know her photo is from photo shoot but I didn't find any Ke$ha's up close photo from photo shoot.
Good video from K$, her best by far (although that isn't too hard to do with her videography) and she's hot in this. Very reminiscent of Britney Spears in many parts, and I would have called her hotter than Gaga IF when she was in the bra + thongs and her ass wasn't so flat. Her body could look a bit better too. If those two things were fixed, she would have been hotter than Gaga has been recently (it's very hard to get hotter than she was during Poker Face).
The triangles in the beginning of the bideos are very BTW-like, it reminds me of it and I like