There is no question.
Stack first, save people later. Unless there are more stacks. Then stacks.
Come on, I expected better of you. http://videomatic3.diskstation.me/~tundrafizz/avatars/NA/208719826.gif
Somehow I feel like Nasus is way more of a midgame champion right now. He really spikes with 400-500 stacks at minute 20-25 and falls off in the lategame.
Why not both?
Late game enemy champions evolve into Q stacks.