i've seen british sea power twice at black cat (once opening for the libertines, i think, and once headlining) and they put on a fantastically out of control performance ... their eyes are all fully dilated (i'll let you determine why) and they're bouncing off the walls, hanging from the pipes, etc ... a guy in a WWI uniform playing drums walks through the crowd at one point, they have foliage all over the stage ... just an amazing energy that worked so well with the kind of out-of-control vibe on their first album ... i haven't heard the new one, but from what i've heard it doesn't seem like it fits in with their crazy live act
i saw kasabian open for the music a couple of months ago and really liked them ... very psychedelic show, lot of jamming, really good stage presence ... they came off as much more mature than i expected for a "buzz band" with one (fairly derivative) album ... that said, their light show nearly blinded me, i had to look away from the stage for large portions of their set
all that said, i think british sea power is the show to be at that night (and the one i'm going to), but that's not taking anything away from kasabian, either ... tough choice ...