Hi there! I went to Markham and my brother went to Roosevelt. They're both indeed excellent schools, both offering the IB, but I would say, IMHO, that Markham is a stricter school. It also depends what you would like for your children... a British approach to education or an American. My brother ended up there because he wanted to go to college in the US, and so wanted to get used to the system. I also went to college in the US, but would have to say the IB was good enough preparation for me.
Markham is English curriculum mixed with Peruvian, Roosevelt is American. Both good schools, one slightly more expensive than the other, and both with very large number of applicants for a very small number of spaces, so you may want to consider other options. I'm afraid most of the private English-speaking schools are the same right now.
I hope this helps a bit!