A 6 pin and an 8 pin connector have the same number of 12V wires feeding them, 3. The reason that cards use 6 pins or 8 pins is because someone decided to spec that a 6 pin should be used for 75W or less of power, and an 8 pin for 150W or less, so when you have a GPU with a 250W draw you get 75W from the slot, 150W from an 8 pin, and put a 6 pin on there for the rest of the power. It is just a convention to help give you an idea of if your PSU actually has enough power for the card, generally adapters are frowned upon and if you have to use them something went wrong somewhere.
In the end, electrically it doesn't make a difference, it is just a convention to help PSU and GPU makers force users to use a PSU that can safely support the GPU.