When to use Nasus and Maokai

Maokai is such a great tank that you can pretty much always use him unless he is against a hard counter in lane. His only weakness as a tank is he doesnt have aoe cc to set up wombo combos.

I honestly dont like nasus. I dont think hes bad, but i think a lot of people at my level of play dont want to play a tank, they want to get a ton of stacks of there q and then carry. Theoretically what you are supposed to do as nasus is use your wither and q to take down carries, but im not sure.