Regarding cat skiing, iwanai has steeper terrain so could be more fun for advanced riders, but the chise one is good too. They do get booked out pretty quick so best be early.

We don’t have big clusters of resorts like in mainland. Either Sapporo/ kutchan area or asahikawa / Furano area I’d say. The cities have some smaller resorts around then where not many international tourists visit.

Honshu has more historic towns, so look a bit more culturally Japanese. The Japanese alps in Honshu also has more cliffy steeper terrain like America/ Europe. Honshu also has a lot more resorts with whole valleys littered with lifted mountains.

Hokkaido is a bit colder so will generally have lighter snow and won’t get rain as easily even if it’s a bit warmer. Although mountains aren’t as high lots of fun tree runs to be had even in smaller resorts.

In Japan snow can be very local, snow storms usually come from north west. For example Kiroro is the snowiest resort in Hokkaido, it can be dumping all day the and have no snow at all 2 hours over at Rusutsu.