I've decided to split 5 days into 3 at RC and 2 at Mitsui. The Mitsui looks absolutely stunning and has private onsen though in June it might not be great.

I wanted to try the newly renovated Westin too but Mitsui at cat 7 is too good to pass (probably moving up to cat 8 in a year or two), while Westin is likely to stay at cat 6 for a while.