Either one should work fine --- the standard pinout for the 6-pin is listed here ---- as you can see the pins are 3 +12V and 3 Ground but pin 2 is normally not used (since the 2 +12V wires can already provide more than the spec 75W - so it is sometimes left not connected !
Not sure exactly -- but figure the 8-pin PCI-e adapter is merely adding 2 additional ground wires to provide grounding for the extra current (up to an additional 75W) that newer cards pull through those same wires - if that gives you any idea !!
EDIT : Found a thread ( Linkie ) I read awhile back explaining the 6-pin vs. 8-pin pinouts that states this :
The #18 leads can carry a max of 10 amps each, so 3 leads @ 12V could theorically 30 Amps or 360 watts.
EDIT #2 and then goes on later to explain the actual spec as follows :
The PCI-E connecter has 6 pins. They are not all used for power. For the 6 pin connector, Pins 1 & 3 are 12V & each can carry 8 Amps. Pin 2 by spec is not connected, although some PSU manufacturers do add a 12V line there. Pins 4 & 6 are Com return lines. Pin5 is Com for sensing. Using 2 lines, you get 12V*8A*2= 192Watts, much over the required 75Watts. With an 8 pin PCI-E connector, 2 Com lines are added (4&8) not a 12V & Com. There, Pins 1,2,3 are 12V, Pin 4 is a Com for the 8 pin connector sensing, Pin 5,7,8 are Com return lines, & pin 6 is for the 6 pin connector sensing. Using that config, 12V*8Amp*3=288 Watts, much over the required 150Watts.