If you are sure that is what you want to practice in, then going to the T14 will open up a lot more opportunities for you.
However, do consider the amount of debt you will have to take on to attend a T14, that the majority of US schools do not offer financial aid to international students nor are you eligible for US gov't student loans, so don't forget to factor in where the Canadian dollar stands right now (i.e. you could end up taking on a lot more debt than you anticipated). You will also have to prove you have the funds for a full year of study at the T14 before you are granted your student visa - loans and scholarships do count towards this figure, although be aware that it will be a much larger sum than tuition alone. The student visa practice for Canadians wanting to study in the US is a lot easier than for other countries and your school will help you with a lot of the minutiae of it.
Also bear in mind the changes going on south of the border and what that could mean for you as you are trying to start your career. You will use up a portion of your optional practical training on your summer jobs, and you will likely be looking for a firm that will hire you on a visa right out of law school. This is not uncommon or unlikely that it could happen for you. Especially since you want to work for an international firm, who are gerneally more liberal about helping you get the right immigration visa.
At this point no one knows what the world will look like in 2020 as you finish your law degree. Even if we assume the legal market stays the same, as a Canadian the prospect of being guaranteed a job/h1b visa/eventual green card is somewhat up in the air right now. With the required salary for an h1b going up to 130,000 USD/year, most people who are starting out in big law will still be okay, but it is not guaranteed that you will get that salary off the bat nor is it guaranteed that the restrictions won't change even more. We also do not know how companies will be looking at hiring practices by 2020, nor do we know how the NAFTA TN visas might change.
It sounds as though you'd like to be able to practice in both countries. It's not easy to cross the border with your law degree in either direction, but from what I understand it's slightly easier to do it going from Canada into the US. If you have a scholarship to the T14, or dual citizenship (or both!) you will have less to worry about.
There is also always the chance that everything goes right for you, and that all of this concern will be irrelevant.