If you ever use or plan to use your chimney, I'd certainly go with the attic. If your worried about it getting stinky, just remember, in the summer the air in the attic is HOT. As the heat becomes greater in the attic, than the outside air, there is a pressure differential created. So.....what I'm trying to get to is, the attic air will always want to escape out through the gable vents on the end of most houses. The sink gets released slowly into the outside air.
Good luck on your grow!