The xgmobile cover has bumps to make the plugs have a bit more width, so that there's some resistance when they're put in. They can't be too thick, otherwise you'll get it stuck (I already made that mistake).
The sdcard cover just plugs into the headphones jack. It has potential to be better, by adding some sort of anchor/resistance above the sd card itself, but it works fine and doesn't move around for me.
