I practiced that for about 2 years when I went to school. Plastic cards tend to be a bit heavier and stiffer than paper. The heavier the Object, the greater the centrifugal force. So they tumble less, so its easier.
That being said, most professional throwing cards you can find are paper. I guess its the same as with cardistry, its a tast thing. Im a big advocate for plastic cards so I might be a bit bias, also I never considered myself really good at throwing so take this with a grain of salt.