Part way through a build, confused by GPU and power supply

I'm part way through building a machine with a gtx 960 and a cs450m. The 960 needs an 8 pin power supply and comes with a 8 pin to 2x 6 pin cable, but the power supply also comes with a 8 pin to 8 pin cable.

Which cable should I use?

As a more general question, are all 6 pin connectors on my power supply the same? Can I plug any6 pin wire into any 6 pin socket?