If you're going to choose between Hakone-no-Yu vs Yunessun for hot springs, and you're going with a bunch of folks who've never been to Japan, then I think you ought to go to Yunessun. Even though the place is atypical relative to standard onsens, and there's an amusement park type aspect to the place, I think your group will feel more comfortable and will be able to enjoy it more for a much longer period of time. Hakone-no-yu is like any small local day-use onsen place... you pop in and out and you don't really spend more than 60-90 minutes. I will say, the food's pretty terrible at Yunessun.