Dell 15R(N5110) Dual channel memory advantage - How to?

I have a Dell 15R i5 windows 7 Home premium laptop assembled in India. The system has 4GB DDR3 RAM mentioned as PC3-10700(667MHz) made by Samsung in CPU-Z and so its 4GB 1333MHz onboard.

I wish to utilise the remaining capacity and thus want to know what is the best possible 4GB option to take advantage of the dual channel memory.

4GB (1333MHz current) + 4GB (Transcend JetRam JM1333KSH-4G - which is 1333MHz, so both RAM running are exactly identical in terms of specifications)

or

4GB (1333MHz current) + 4GB (Kingston KTD-L3CL/4GFR - which is 1600MHz and so will downclock)

Does dual channel memory provide any additional noticeable advantage and will it work if one of the RAM module is downclocked ? How to identify whether the dual channel memory is working or not?