Both, but added at different times. The white and light green bottoms behave like any mild onion: a building block of flavor for almost any dish. Gently sauté the bottoms of the green onions first before adding other ingredients. The dark green tops are super crisp and fresh (think of them like chives on steroids). Save these for last, adding to a stir-fry just as you pull it off the heat or sprinkling over baked potatoes, chilis, or tacos. Trim and discard the root end and very top of the green onion. Then, thinly slice or chop the remaining part of the onion, separating into two piles.
Or, instead of chopping, try Grilled Green Onions with Lemon and Parmesan. The charred, tender, and slightly sweet stalks are a revelation.