December is much better than June, as many had pointed out.
With 4 people, a car is better than trains.
To stay in Marriott hotels in Japan, I'd suggestion not to stay in Tokyo. Instead, look at Yokohama (assuming you had a car/ or not), cheaper and more convenient. If you want to go to visit Tokyo, then use trains.
Marriott had added many new properties in small cities (yet close to bigger ones). I'd be looking at them if I had a car.

Near Christmas time, all cities are very festive.