Thanks robby and bvukich. I will try manual and 50mm prime, though I'll miss the 15-85 versatility. I will post some photos for your judgement and critique as it will help me understand my bads and my goods.
Lastly I would like to seek advice on which focusing and metering modes to use. I've seen a tutorial advising to use the AI Servo as it re-focuses during subject motion. Also should I use spot metering when I shoot single dancer and evaluative when shooting a group?
A few thoughts from my experience with dance.
1. To answer your questions about focus and metering modes, if you're in manual as you've said you're going to try, it's a moot point. The AF and metering only tell the camera what aperture and/or shutter to use. In manual mode, you've done both of those. If you do not use manual, my practice is AV mode selecting the fastest aperture I can manage to get the fastest possible shutter speed. As has already been said, you may also want to doctor up your ISO. The 50mm at f/1.8 will give you a fast shutter speed, but it's not very sharp and the DOF may be too narrow to be effective. I'd try selecting a F/2.8 and see what kind of shutter speeds it gives you -- then adjust ISO, etc. from there.
2. If you're not doing manual, use center point only and whatever that camera's version of AI/Servo is. There's too much going on in dance to fool around with getting other focus points involved. Put the spot on what you want and shoot it. As for metering, I've usually used partial as it's a larger area than spot, but it doesn't bring in a lot of background stuff that can screw up the focus of your picture. But stage lighting is tricky and varies a lot, so experiment. Do a lot of checking with this stuff in the beginning so you can make adjustments as needed.
3. I agree the 50mm is really the only option here, so be prepared to move, if you can. Be prepared to be assertive and get to where you need to be to get your pictures -- those people who are only watching are only watching; let them accommodate you as what you're doing has a higher priority. Oh, and if you can use the flash, probably not a good idea. First, it's new and you never want to experiment when your life is on the line, and second it is really distracting to everyone concerned. I've never shot dance where they even dreamed of allowing flash.
4. The point about "creatively" slowing to use blur to communicate motion is a good one. I would do some of that, but I would also want to pointedly choose focus points that will be well focused. One thing to do in dance is to look for the quiet moments. No matter how fast the movement, there are always points when the motion stops -- anticipate these and be prepared to press that button at that exact second. For example, a dancer may whirl around and stop instantly at some point -- the head will be still at that point while a dress may still be catching up -- be ready for this. You can have a face in perfect focus while communicating motion in the colorful dress material. That kind of contrast gives you a sure winning picture. Also, this is where burst shooting comes in -- you'll maximize your chances of getting just the right shot. That also means you'll need a fast card and lots of storage. When I shoot ballet, 1000 to 2000 images is common for an event.
5. Get there early and size up the shooting situation. Find the best place for you to be with the equipment you're using. You may want to plan on moving just once if moving is difficult. From one pov you may shoot tight faces, torsos, etc. From the other pov get groups.
Fun, huh? Have a great time and get some killer pictures!