My standard for evaluating cabbage greens: fry some bacon ends and pieces, add Polish sausage (or similar) half moons, when done and browning toss in stems and some water, when half done, toss in leaves.
Collards failed dismally. They're tough and take forever to cook, and when they're done they're not worth the bother - slave food - dull and nearly inedible ("builds character").
Mustard greens are one of my favorites, along with Chard and Italian broccoli (Chinese broccoli is even more tender and leafy but not available in Red States (or even some Blue States).
I've got a big head of Gai Choy out in the garage fridge. It'a an Asian mustard cabbage with a sharp mustardy taste and I'm sure it will work just fine (Note: most Asian cabbages should be cooked rather briefly).
[image|http://www.clovegarden.com/ajg/cb_gaichoy1.jpg||||]