I actually stayed in these exact two hotels (grand hyatt in kauai, princeville resort) in 2005, so I can comment on them (pre-rennovation).

I paid more for the princeville but preferred it, generally. The princeville had fewer guests, had a spectacular view, a nice private beach, ...

The GH was nice but was larger and was set up for conventions. So, everything was an add-on cost to the point of annoyance (to me). Getting towels for the pools was extra, for example. I had a great balcony room at the GH and spent one afternoon reading on it and watching the whales go up and down the coast. the rooms were cheaper and perhaps cost less overall, but I would not say the two properties are equivalent, in my mind.

Also note that if you want to go SEE the Na Pali coast up close, all of the whale watching tours now leave out of the S/SW part of the island - you can't drive counter-clockwise from Princeville to get down there, so you have to drive back down through Lihue (apprently they used to have the boats near princeville). Had I known, I would have booked a hotel down there for that day as it was a good hour+ drive to get there. I still have that picture on my wall of the coastline, so I recommend a boat ride out there if that is your thing.

Lastly, I am returning in July and will stay at the princeville, if that means anything in terms of repeat business. My wife and I had a great time there, and we are looking forward to some R&R when we return.

I hope that helps,

Conroy