I stayed at both the Mauna Lani Bay and the Hilton Waikoloa last week (and I went to Hapuna Beach, but not the hotel). Of the two I stayed at, I'd go with the Hilton next time. However the Hilton doesn't really have a real beach, it's on rocky lava however they do have a manmade lagoon which is decent. That said, if if it were me in your shoes, I'd probably stay at the Hapuna Beach Prince. It's right there on the best beach on the Big Island. It's a no-brainer. And although I've never been in the hotel, I can't imagine any hotel in Hawaii being too stuffy. People come there to relax, not dress up all nice to impress people. I would be really surprised if Jeans and t-shirts were TOO casual there. I just wear a swimsuit and tank-top 90% of the time I'm in Hawaii and I never/rarely feel inappropriately dressed. Definitely go for the Hapuna Beach Prince if you wanna be on a nice beach.......^