The notion that water behaves differently in northern and southern hemisphere basins is a nice little earner for smart operators living on the equator. In reality, the direction in which the water goes down the plughole is determined by several factors, such as the shape of the basin, and the way the water is moving before the plug is removed, etc. The position of the equator has little effect at all.
There is an African country near the equator where entrepreneurs have set up two toilets, one just north of the equator, the other just south of it. For a fee, they will allegedly demonstrate that the toilets flush in opposite directions. It is only for show, however; there is no real effect. Yes, there is such a thing as the Coriolis effect, but it is not enough to dominate the flushing of a toilet--and the effect is weakest at the equator.
Essentially, the "Coralis" effect is not really visible in something as small as a sink drain or toilet. For the equator, the answer is: either direction.
Here are some good sources:
http://jcbmac.chem.brown.edu/scissorsHtml/liquids/bathtub.html)
http://www.sciam.com/askexpert/physics/physics20.html)