The Ripjaws X is the newest RAM of the three. It will have the newest circuit board and newest chips on that board.
The other two are older and have likely been sitting in a warehouse for a while.
Other than that, they are all the same. The brand of the chips on the circuit board might be different, but they are all using the same timings and voltage. Any of them would work for Sandy Bridge. I would choose the Ripjaws X as they are the newest, but that's me.
Edit: The one in the middle isn't a dual-channel kit -- it's just one 4GB stick. That's why it is $10 off.