Momofuku noodle bar or Ssam Bar
I'm partial to Noodle bar because it's cheaper, and I love to sit at the bar (kitchen) and order the prix fixe. It's louder and more bustling, whereas Ssam is darker and more relaxed. Ssam is better if you're ready to spend freely and order a lot of different dishes. The prix fixe at Ssam is always the same, but the prix fixe at noodle bar has some really fun dishes that may only be on the menu for one day. And yeah, those slushies at Noodle Bar are irresistible, esp. the watermelon.
Not sure there's a good answer for this. I think most CHers think that Ssam Bar is a superior restaurant, and it's certainly the one with a wider variety on offer - you can come closer to putting together a mini-Momofuku Ko meal there if you set your mind to it. But Noodle Bar is probably easier to make lunch out of, simply because you can order a single noodle or curry dish (plus, of course, the pork buns) and be done with it. Just ordering a single 'entree' dish at Ssam Bar would almost be to miss the point.