Here is what I wrote about Kauai vs STJ after our trip to HI in 2012:
Impressions of Kauai vs STJ: The biggest and most obvious difference between these two places in their size. I knew before we went that Kauai was pretty big, and but it still surprised me HOW big it is in comparison to the islands of the VIs, so much so that it is almost too much to take in on your first trip LOL.
The size of the mountains, the topography, the different environments, the dynamic weather, the sweep of the shorelines, the amazing vistas, the lushness of the north shore, and the size of the waves etc. are all pretty amazing, even mind blowing. It’s like St. John on steroids LOL. On the down side - - more cars, more people, more traffic (but not unmanageable).
Indeed the size difference is substantial. Google tells me Kauai is 562 sq miles, STJ is only 28 sq miles (!), and all of the islands of the BVI add up to 59 sq miles.
The from the beach snorkeling in the VIs is way better than Kauai, the water is warmer in the Caribbean, and nothing beats those colors of the Caribbean water. On our first trip to the VIs in 2005, on the ride in from the airport, my daughter kept exclaiming “Is that the ocean?!? Is that the color of the water?!?” Not the same in Kauai.
Market food prices in the VIs may even be higher than Kauai. In Kauai, stuff can be had at the Safeway at reasonable prices. There is even a Costco on the island.
Re other costs, sure airfare will be much more $$$ for most, but car rental and nice rentals with pretty great views can be way cheaper vs STJ, and that can offset the AF cost.
Re eating out, we were VERY impressed by the many kinds high quality of the fish we had in Kauai and the good options provided by the food trucks and hole-in-the-wall joints. Great fish tacos everywhere. Many more good options for lunch. Caribe and Presidente beer is way better - was not a big fan of some of the local HI brews I tried. But was also able to find Cruzan for like $7.99, so that made mixing up the homemade Mai Tais reasonable.
Folks in Kauai are quite friendly - people are very nice and real laid back. No disapproval if you forget to say “aloha!” LOL.
Never saw so many old hippies in one place and old guys with gray hair down to their shoulders, esp on the North shore. I thought hey maybe I would do that too if I lived there LOL. I also liked what I call the “surfer” mentality of the people and the “surfer” vibe of the small towns – way more charming than those in California. The Kauai people appear to have much more environmental awareness too - not a plastic bag to be found on the island (just a small example).
In general, St. John seems to have a more "tropical" feel to it than Kauai as STJ is more humid, STJ doesn't have the same frequency of trade winds blowing. STJ has the tree frogs singing their night songs and has that kind of musty tropical smell, but both also have the crowing roosters