Deciding between a car or van for Japan trip

Hi,

I need some help deciding on whether to go the route of renting a van or a car for a 3 wk trip to Japan. My partner and I are renting one to explore the country side and be able to go into the mountains for maybe multiple days at a time. We are planning to go into end of June to early July. I am wondering if we are going to be miserable with the temps and should just go with a car and stay at hostels instead.