Aperture should be the first thing you set when testing your pictures. Start with a middle range of 5.6 just so that if you have any group shots to take, you'll have all your subjects in focus. Remember that the lower the number the more light will come in from your flash but too low and your gonna get some people out of focus. So that's where ISO comes in handy. Try not to go past 1600, since it can look rather harsh on your images mainly due to the strong light source.
As for flash settings, best to stay with ETTL. A night club or performance event can be so dynamic that you won't have time to constantly change your flash settings. It's difficult enough being on manual and playing around with your camera settings. Make it as simple as you can.