Is it difficult to get the unit you want at this resort during ski season? I appreciate any info!
The difficulty depends on which particular time of ski season you are looking at. My experience is about Whistler area resorts in general; there may be some special situations with Cascade that I'm not aware of.
Ski season at Whistler typically starts sometime around Thanksgiving. The period after Thanksgiving and leading up to Christmas often has availability. Conditions can also be marginal, depending on whether snows continue into early December.
Starting the week before Christmas and extending through New Years, demand is very high. January weeks show more availability, particularly the first couple of weeks. Demand starts picking up near the end of January, and February through mid-March is the peak of the ski season. Presidents Day week is probably the highest demand week of the season. Availability starts increasing again in mid-March, and gets progressively better into April. Spring skiing season starts sometime mid-late April.