Kauai is a smaller island with lots of natural beauty. You can easily spend 3 days sightseeing and/or enjoying the beaches. We've already booked our third trip in three years next winter in Kauai - we can't get enough!
Have a great time!
Sort of like asking someone to pick their favorite child! That said, if you're looking for natural sights, then I'd choose Kauai. Awesome hiking on the NaPali coast, countless waterfalls, the Waimea Canyon, river kayaking. You really need to take a helicopter ride to appreciate it all, so much is unreachable by car.
I'd give Maui an edge for beaches, the road to Hana drive and the sunrise at Haleakala.
Either one's a winner, but in 3 days you could see a lot of Kauai. Just enough to whet your appetite for a return trip!
I think Kauai is a good choicefor just three days. Good choice on Lanikai, too! You'll love it-
Gosh, flip a coin. You really can't go wrong. Kaui is more natural and undeveloped, so it's a big contrast from Oahu that way. But Maui is a bigger contrast geologically. It has plenty of natural beauty, but a different look than Oahu and Kaui because it's a newer island (Kauai & Oahu are the oldest).
I'd say Kauai, but then again I'm biased towards it. What are you looking to do for those 3 days ? Maui has more things going on in terms of shopping, restaurants, tourist attractions fancy hotels and great beaches. But with that you've got more cars, more people and (to me) more hassle.
Kauai is more laid back and has much more natural beauty, but shopping is more Macy's than Gucci. It has great hiking and some very awesome beaches though not all of them are swimmable. But you can find ones that are not crowded at all.
The only thing that I like about Maui more than Kauai is that Maui has more and often better roads for bicycling.
Either choice is a winner though.
Kauai! Have fun!
Kaua has more natual sights and less shopping than Maui. Hope this helps.