Hello Belles,
In my own experience, I have found that most of my MA cohort at York is left-leaning, especially in political theory and IR. I think if you have plans to go on with a PhD, doing an MA here would expose you to critical political science, and would likely inform your decision in terms of whether or not you would want to continue down this path or to move to more mainstream political science. That said, York is probably one of the best departments in all of North America for this kind of work.
For me, I was drawn to left-leaning poli sci from the beginning of my undergrad, and so chose to go to York. The problem with York's poli sci, is that there are no real faculty members that do "mainstream" political science. I am sure that they are all aware of mainstream political science, but it isn't their strength.
York has a fantastic environment for learning, at least in the graduate level. (I have heard horrendous things about York undergrads). One shocker that I wish I had known going into York's program is that they do not have any formal methods/stats courses or sequences. (This may make it difficult for you to transition into political science programs in the states, as most will have a methods/stats requirement) [This is less so in Canada, where Political Science is not nearly as quantitative, but it is still something to consider]
All of this to say, the MA at York does open doors in America and Canada, but PhD admissions are a different demon altogether.
If you have more questions, let me know!