The risers fitted with the molex 4 pin connector take +12V and +5V from the molex. The risers fitted with the 6 pin power connector only get +12V from the connector so need an on board regulator to produce the +5V needed by the GPU. The 6 pin type is best suited for when you have a separate power supply for the GPU's as there is then no possibility of connecting 2 different power supplies to the GPU at once. The 6 pin types have more components on them to give the +5V supply and therefore have more scope for failure.