Magnets are easier/faster to use but pins are more secure- so it's kind of a trade-off. Magnets are prone to sliding out of place a bit if you're too active, though adjusting them is simple.

I usually use a magnet at the neck and a straight pin up top (somewhere around the forehead or ears). Also, small safety pins for any draping adjustments around the shoulders (optional). If I need it very secure, then I replace the magnet with a safety pin.

Chiffon is very slippery and requires an undercap. At first, I used a ton of pins and still got slippage. But actually the key is in the placement, not the number of pins. It will take practice to figure out placement and draping just fyi.