For those who have experience with both, which do you prefer and why? Or are they equally great? I realize there can be quite of bit of variation from island to island within these two areas, but if you were to compare the best of each, which would win?

I don't believe there's a simple answer to your question. The best beaches in the Caribbean and the best beaches in Hawaii are outstanding, but in slightly different ways. I don't think it's possible to generalize and state that one group is better. Some individuals will value different aspects than others.
Poipu, Makena, Hapuna, and Hanauma beaches in Hawaii are spectacular.
St. Maarten, Antigua, St. John and the Bahamas all have beaches with equal appeal. The sand at the best Caribbean beaches I've been to has been a little softer, and the water in the Caribbean is warmer and more turquoise in color in many places.
But the amenities on the islands, quality of the food, variety, activities, and quality of the resorts affects my opinion as well since I'm not only choosing beaches, but a vacation package.

Although I love Hawaii, when it comes to beaches I'd choose the Caribbean, hands down. The waters are calmer, the sands are softer, and the water is an incredible shade of blue that I've never seen replicated anywhere else. Also, Caribbean beaches are more likely to be white sand, while the sand on Hawaiian beaches tends to be more brown.

Caribbean, by a landslide.

I'm not a beach connoisseur so I just have to ask Mike what the difference is between white sand and brown sand. Does the color indicate the texture?

Can' compare but I can tell you on Kauai, Big Island, Maui, Oahu you have tremendous diversity with indoor outdoor activity. I can't imagine anything but French Polynesia rivals Hawaii's beauty. Hawaii also offers a rich glimpse into Asian culture which I find special. Food food food is wonderful and I could go on but you need to make these comparisons.

Not technically the Caribbean, but the most beautiful beaches in my world are Eleuthera and Harbour Island in the Bahamas. Deserted, and - get this - PINK!!! (A tanned woman looks so good in pink!
Cheers!

White sand is finer than brown sand.

Caribbean because of warmer and bluer water, white sand is softer, love their music, too.

Agree with the overall "sandage" being nicer in the Caribbe but the music in Caribbe mo' bettah' in Hawaii Caribbe?

No haaaWayyyyy, bruddah. Some good slack key and a good "talk story" no ka oi!
Granted "Jawaiian" is cool too.

Please, everyone, go to the Caribbean! Leave Hawaii alone. The beaches in Hawaii are terrible!!!!

ummm....methinks you've been in the sun a bit too long there, worshipper.
As an overall vacation experience, the Caribbean isn't in the same league as Hawaii. The sand is better in some places, but man does not live by sand alone. Must eat and sleep and sightsee also.

Hawaii is definitely the superior destination, BUT, in my opinion, the beaches of the Caribbean ARE nicer: White sand (simply more esthetically pleasing to the eye), calm, clear, and warm water and, of course, much prettier shades of blue/green!