I don't know Knox at all.
I have to say that I really really like Wheaton. I used to work nearby, and my son went to accepted students day at Wheaton in 2010. The first year Italian teacher -- we sat in on her class -- is awesome. I have NO Italian (but Spanish and Latin and Russian) and I was able to participate by the end of the class.
There's lots and lots to like about Wheaton. I got the feeling of a place that an ambitious student could really get a lot out of -- good teacher, nurturing, lots of opportunities.