It really depends what you want to do. Both have great programs, respectively. I think that both schools' full-time day programs are more like "cosmetic techniques and management" or something like that. I know a couple of people that work at MAC now that have taken the Sheridan course.
But Seneca College also has a part-time certificate program called "Special Effects Makeup Artistry", and this is the program that I have heard MANY raves about, from teachers, past students and makeup artists. I actually am taking this course now, I just had my first class last Saturday. My teacher used to teach at Complections. I first wanted to take this class last year, but I couldn't get in, I was placed on waiting list. Someone in my class has actually been trying to get in for 3 years! This course is always in high demand, I recommend registering ASAP on the first day registration opens (especially if you want the September/fall start). Since I couldn't get in, I took Centennial College's Makeup Techniques part time certificate program last September, which I will be completing this June. It's fun, but very basic, which is why I still want to complete Seneca's course. I wish I could tell you more about it, but since I've only gone through my first class, it's hard to say =P But I know we will be covering runway, theatre, bald caps, etc. I spoke to someone who has completed it, and she said in the later courses, you actually have to get a model from an agency for your photshoots/exams. Anyway, sorry about the novel, but this is the first time I'm actually excited about school, LOL! There is more information here:
http://www.senecac.on.ca/parttime/pip-makeup.html They even list the course outlines as well. I hope that helps even a bit!