24" is a medium depth tank. Anything over is considered fairly deep. For 24" VHO would be fine for anything but SPS and certain clams. But, in your original post you mentioned wanting to be able to "keep any type of coral thriving, not just surviving". IMO the only way to do this is to go with MH's. For 24" you would be fine with 250 watts. If you really want to be able to keep anything, you could go with 400. But 250 should be plenty. With halides typically you want one bulb per 24" of tank length. So for a 6' tank you would want 3 bulbs. HTH