Sand is definitely a better choice than crushed coral. Not only does sand have better buffering capacity than crushed coral, but the pieces of crushed coral are also big enough that detritus and gunk gets trapped in it, which leads to nitrate problems. And thus, algae problems. That, and animals that dig or bury can't live in crushed coral.
If you put sand on top of crushed coral, the small-grained sand will eventually just make its way to the bottom of the tank, leaving the crushed coral on top, and you'll end up with the same problems as if you just used crushed coral.
It usually takes a few days for the water to clear up after adding sand.