as far as I know, these are two completely different use cases. With a site=piste relation, all ski slopes of a ski area are collected in one relation.
With an outline of landuse=winter_sports you define the geographical area of the ski area, independent of the slopes.
Assuming you know where a ski area extends on the ground, but not exactly where the slopes are, you can create an area with landuse=winter_sports. If all the slopes are already mapped individually, you can summarize them in a site=slope relation. But in principle, both can be used at the same time.
Checking for the two types is the way to go. Beware that some landuse=winter_sport are not resorts, but also sometimes a single piste extent, or a biathlon shooting range.