Asus Maximus VII Ranger[4x4GB or 2x8GB]

Hello i ordered Asus Maximus VII Ranger and Intel I5 4670K CPU
what i am asking is i already have corsair vengence ram 2x4GB
I want my ram to increase to 16GB so what is best to buy 2 more stick of 4GB or 2x8GB is faster?
 
2X8 GB will be faster as the memory runs in dual channel, so 4X4GB will basically be 2 sets of dual channel memory. If you want to increase your memory capacity, you'll have to get the very same pair of sticks as you have, so that there are no incompatibility issues.

But mind you, memory is notorious for not working even with exact same copies at times, so if you can get 2X8GB , then I'd prefer that over 4X4GB.
 


i just want to expand memory to 16GB and yes the other 2 ram will be will be the same model