Go with black. A lot of fish will change their colors slightly to try to match the substrate. If your substrate is white, that will make their colors washed out. If your substrate is dark, the opposite will happen, they will darken their colors to match their darker substrate and you will get more vibrant coloring from the exact same fish.
White substrates also tend to reflect light a bit, which can make fish uncomfortable because they will be getting some light glare from all directions - from the glass tank walls, the actual light at the top of the tank, and the reflection of light from the pale substrate. This effect will not happen with darker substrates, so fish tend to be more comfortable with them. If you are having problems with algae you may already have too much light on the aquarium (particularly if your algae is green, which tends to grow when there is excess light), so the dark substrate not reflecting could be particularly important in that case.
Your Cories will just love the sand. Make sure that whatever kind of sand you get is smooth rather than rough, as they will constantly be pressing their tender abdomens against it.