It is simple maths - 256 is not as good as 512. Having two is not even going to be as good as one if the numbers were equal. Dual Channel is not going to make it that much faster - and it will help both two 128 and one 512 equally; therefore 512 is faster and better.
I'm not entirely sure what the question here is? Dual-Channel just means two channels can be sent into and out of the RAM - it doesn't care whether there is one, two, three, or four sticks in.
A Knight is sworn to Honour. His heart knows only Virtue. His blade defends the helpless. His might upholds the Weak. His word speaks only truth. His wrath undoes the Wicked.