hey vp some advice please-

I am buying a haul of pokemon games for DS
I bought

HeartGold gen 2
AlphaSapphire gen 3
Diamond AND Pearl gen 4
I am going to sell one of them from gen 4 because I don't need both, is there any difference? Which is better/should I keep?

I am going to buy gen 5, white or black.
What is the point of White 2 and Black 2? Is there any point getting them?
(I've never played after gen 4)

should I get gen 5? X and Y?