The obvious question is how far away are you.

Macro/wide angle - don't be silly!! Leave at home.

If you are near enough to get the shot you want on 24-85mm then I guess that will have a wider aperture so I would try that first. If you are miles away it will have to be the telephoto.

Either way, set to Aperture Priority and fix the lens at max aperture. That way you will get fastest shutter speed and minimise camera shake (keep ttl metering on). Don't worry about depth of field - you will most likely have the lens at inf distance, so this will not be an issue. Forget about flash - it will not make a scrap of difference. Use a good lens hood to avoid any flare from side lighting.

Difficult conditions to get quality shots, but remember to take something (eg bean bag) to prop your kit on stably and focus steadily.

PS enjoy the gig - don't waste ALL your time looking for that 'perfect' shot!!!!

PPS Check out the lighting set of any number before trying a shoot - if the stage has spots sweeping the audience be very careful you don't catch them when looking through the viewfinder - unless you want blinding in that eye of course.