<p>As you say, your exposure to both schools has been limited. Based on everything I have seen and heard from my kid about NU over the last couple of years, I think you might have gotten a misleading impression about the social atmosphere and social life there. I think that both are vibrant. My kid is a serious student and works very hard, but is very social and still has a great social life and does plenty of stuff unrelated to academics.
Frankly, I do not think you can lose. In my personal opinion, you are choosing between really good (ND) and even better (NU).
Good luck!</p>