For Hokkaido it's strongly recommended to use Docomo backed service as they have much better coverage. Previously when I roam on Softbank's network to some middle of no-where place, Docomo still shows connection but none for Softbank

I believe their premium service is based on au which should be better than Y! mobile (previously eMobile), that my friend who used to work in Niseko have better reception with au than docomo indoor