This thread is the latest eye-opener to me in terms of cross-national differences of things I assumed were standard across the country. Slurpees were/are an absolutely ubiquitous thing throughout the Vancouver area, both for pre-teens making their first journeys to 7-11 on their own and adults cooling off on a summer walk. Macs Frosters played the same role and are the same thing.
Slush Puppies I saw pretty much exclusively in community ice rinks and movie theatres, and I'll second the point that they're more like snow cones than slushies.
Didn't see any noticeable difference in Slurpee consumption when I was in Winnipeg. I was amused however, cause as a teenager down the wikipedia rabbit hole I actually read that Winnipeg was the #1 Slurpee consumer per capita and thought it was a funny obscure fact. I was genuinely shocked to find out that EVERYONE in Winnipeg is aware of this. I was chastised more than once for drinking Macs Frosters, as that could've been a 7-11 Slurpee contributing to the city maintaining its place