What are you planning to do? I honestly think 1600 is the sweet spot between price and speed... but if I had to choose between the 2 you mention I would choose 2133
8GB DDR3-1600 or 16GB DDR3-1600 is all you need 8GB DDR3-2133 is good too but a bit more expensive than 1600 RAM speed doesn't make that big of a difference compared to other things on your PC.