Just my experience but when I book Villa Fontaine the onsen is an optional additional charge, discounted for hotel guests. Because I don't book the onsen when I check in the kiosk spits out a discount coupon for it anyway.

moondog I have used Villa Fontaine several times this year. For me it was always cheaper than Royal Park. Regardless of which hotel you choose, try checking hopper and trivago to make sure you get the best rates. Hopper has some good rates for Japan, just don't accept any of their add-on options. For the Villa Fontaine, pay attention to the checkout time, they offer slightly different rates for checkout at 9 am and a tad more to checkout at 11 am. Keep a copy handy, one time they thought my price of 110 was too low and had to be a 9 am checkout. It wasn't. There is a 24 hour convenience store in the basement which you will walk by as you go to the hotel which is, well, convenient.