I've seen O, Mystere (2x), and Love. I like them in that order. Love is a great show, especially if you like the music. O is more of a spectacle, more dazzling.
I just saw LOVE last night. I saw O a few months ago.
I absolutely loved both, but I'd definitely give O the edge. It was just absolutely mind blowing and totally unique/not something you can really experience anywhere else, whereas LOVE was a gorgeous celebration of the music with more intricate dancing and artistry, but not quite as much on the WOW factor.
I feel as though I can't trust people who like Ka. When I saw it I was like WTF is this crap. If I wanted to see acting I would've gone to a play. DO SOME ACROBATICS!
It shouldn't be labeled a Cirque show. It's boring as Fuck with zero acrobatics/cirque stuff.