Each module has a physical 64 bit wide data bus. In single channel that's all you get. But in dual channel the controller makes the data bus appear to be twice as wide or 128 bits so it can read/write twice as much data at the same time.
2x4 gb will give more speed, but if you only have 2 memory slots on the motherboard I would go for 1x8. But that would be the only reason as 2x4 will be much better overall.
Each module has a physical 64 bit wide data bus. In single channel that's all you get. But in dual channel the controller makes the data bus appear to be twice as wide or 128 bits so it can read/write twice as much data at the same time.