both have pro and con....
but just to let you know....
collapsible grey card need to take out to use and keep it after you done with it, after sometime, either you decide not to use them or spoil already..
exposdisc is handy, just put in your pocket, can take out any time you want it, but easy to get lost, btw, have you lost a lens cap before? that is like a two hundred bucks lens cap you know?