thoughts on PTC resettable fuses to protect USB port

I always find this confusing, don't think I've ordered the same thing twice.

Do we go with a low ohm part that will Hold at 500mA and trip at something higher? Or hold lower, like 250mA, and trip closer to 500mA?

Hold 500mA, trip at 1A http://www.digikey.com/product-detail/en/0ZCC0050FF2C/507-1361-1-ND/1560228 http://www.digikey.com/product-detail/en/1812L050PR/F2150CT-ND/267852

Hold 350mA, trip at 700mA http://www.digikey.com/product-detail/en/0ZCC0035FF2C/507-1497-1-ND/2165326

Hold 250mA, trip at 500mA http://www.digikey.com/product-detail/en/0ZCC0035FF2C/507-1497-1-ND/2165326