I think profs are preferable but admissions knows that sometimes TAs are who you interact with on a daily basis and who actually know your work. So it wouldn't be a bad thing especially if it's a glowing letter of recommendation.
@sx23 no but I feel dumb for sending them an email. They clearly say on their website that they waive the fee for any application received between 9/1 and 12/1 http://www.law.northwestern.edu/admissions/faq/faqjd.html