Depends.
If you're planning to use Kaby Lake's integrated graphics, dual channel memory should prove more usefull and give you slightly better performance. It basically doubles the memory bandwith.
If you will be using dedicated graphics card and plan to use the pc for gaming, you won't notice much difference. Mostly where you would see small difference is in synthetic tasks, giving an edge to dual channel memory.
Since the ammount of memory is the same and 2400MHz is only about 11% faster to 2133MHz, I would go for 2x4GB 2133MHz because of dual channel configuartion.