I think as a person watching the show not knowing Britney or Demi, I would think Demi is a much better judge who is more relatable, natural and funny. She doesn't seem to be trying too hard and she just seems comfortable in general.
Britney was much better than I thought she would be (my expectations were pretty low...) but it seems like they might have edited her to look much better than she actually is. I guess we will have to wait for the live shows to see for sure. However, regardless of editing she was more articulate and comfortable than I thought she would be.