My daughter does math and language through Johns Hopkins CTY online. Up through the 6th grade, the math comes from Stanford's EPGY. After that, beginning with Prealgebra, the math comes from Thinkwell. You can see some of their videos on YouTube to see what they're like.
My daughter liked the EPGY format better, but I think they are both well done. There is good teacher support through JHU.
I don't know anything about Northwestern's program, I'm afraid.
Hope this helps.
BTW-I use CTY math & language for my homeschooling. It's kind of expensive, but very worth it, in my opinion.
