Congrats!
I would strongly suggest you take up U Mass!!
U Mass is ranked 8th for AI by US News.They have a very strong presence in Reinforcement Learning and NLP and IR.
Their NLP strength will be most relevant for you in IR.
http://grad-schools.usnews.rankingsandreviews.com/best-graduate-schools/top-science-schools/artificial-intelligence-rankings
Waterloo is very well respected within Canada but I seriously doubt its international impact as compared to U Mass Amherst.
Waterloo is considered the best for CS at UG level owing to their strong Co-Op programs. But, at grad level, U Mass Amherst is much better.
You just can't beat a top 10(in a particular field) US School in terms of career opportunities. Having said that, if you are really interested in some specific prof, Waterloo may become your first choice.
All the best!