Cold on top, but plan to "swirl" the wort (even just one lap around the pot) while chilling every few minutes. If you can do that in a sanitary fashion, the wort will chill a ton faster.
Without the swirl, the wort gets cold by the coils, and takes a long time to transfer the heat to the rest of the wort. By agitating the wort in some fashion, the move the hot wort to the coils for maximum effectiveness.
Also, if you are building an immersion chiller, think about putting hose couplings on both the in and out. I use my hot water coming out of my chiller to clean up while I am chilling. With a hose coupler on the out side as well, I use a garden hose a safe distance away from the pot to clean.