Kale is my favorite green - First of all, it's beautiful, however it's a plant that thrives i cooler weather. The local season will peak in September or October. Kale has tough leaves and a slightly biting flavor. Not everyone likes it raw, but I find it wonderful broken into a salad. Kale can be slightly steamed, and eaten plain or added to casseroles.
Collards are the toughies of the crucifer greens.
They're thick, dusty green leaves, broad and long. We have collards much of the late summer. They're a little more bitter than Kale, and are rarely eaten raw. They're usually steamed or stir fried, served with vinegar. Mustard greens are sharp and tasty - with a real bite.