Yeah, I'm afraid that, as Mobey states you'll get differing opinions on all schools from different grads.
The advice that I do have is that if you are based out of BC and want to work in BC, its not too bad of an idea to pick a school in Alberta, as its close logistically, and registration transfers over fairly easily. Additionally, if you're thinking of taking school in Alberta, I'd also recommend you take a look at PMA, they have classes out of Sherwood park and Calgary.As a Paramedic in Southern Alberta, I get most of my students from either PMA or SAIT. There doesn't seem to be a particular difference in the quality of students each produces, I find its more the attitude of the individual that makes the difference.
As you mentioned, the selection process at SAIT doesn't seem to be that strict, they also have bigger classes and go to school full-time monday-friday. PMA has a rotation of classes that is two weeks on two weeks off, smaller classes, stricter selection process and a shorter length for the program. NAIT I can't really speak to, as I don't get any students from that program.
So... its up to you to decide what the best fit for you is. I think the best thing to do would be to go to an open house and/or talk to the instructors at each school, see if you like the facilities and also the philosophy of the teaching staff there; ask what their pass rate for ACP exams. I would really not put too much stock in one off opinions of ex student from each school, as they tend to reflect personal bias, rather than the reality of the institution.