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I want to get the best troop choices to make up 400 points of army from grey knights and ultramarines

I would suggest scouts. They're (relatively) cheap and with the UM tactics they can do a surprising amount of damage. But, my question is, why are you bringing both Marines and Grey Knights at 400 pts? I would honestly say just go with one or the other.

I would stick with pure scouts from with Ultramarine tactics. Quantity has it's own quality. Grey knights pump out a decent offense but are still t4, 3+ dudes. At 400 points each one you lose is a is a blow at 20 points a model.