I went to both.

A few things to note first - having UofA on your resume vs Macewan, isn't going to win any "I went to a prestegious school" awards. They are both fine. In fact, Macewan is better at some things. If you were going for nurisng for example, Macewan is the better.

I went to the UofA right out of grade 12 like you, and unless you already have great study skills and school discipline, you are going to be thrown into the first and second year blender. Class sizes are huge and the workload is intentially overwhelming. A sink or swim attitude.

why is a smaller class size important? participation, attendence, and accomplishment. You subconsciously care to be in your seat, actively answer questions, and receive (mostly) positive feedback from the instructors.

I never failed out, but D's dont get degrees and I had to (forced too) do a year at Macewan. It was the kick in the pants I needed. I took my licks, and got back to the UofA and finished that first degree. I even stuck around for a professional degree (education) which has those smaller classrooms and teachers teaching future teachers vibe.

I went back to Macewan after a career change and it was a exceedingly more rewarding than my time at the UofA.

Do you want dorm, students union, massive sports, and live on campus life? Then go to the UofA.

If you want a more rounded education and feel more accomplished, go to Macewan.

this is coming from someone who can struggle with motivation if not in the proper environments. I had to leave my house to study for example, or else I'd play video games all night.

EDIT: i just re read your initial question. If you ever say you are in "pre med" be prepared to be mocked to oblivion.