The resorts aren't that much different.
The islands are tremendously different - so I'd choose the island first:
Oahu - Honolulu/Waikiki offers a lot to do and both tourist and historical attractions, but it is a bustling BIG city. The population of Oahu is greater than all the other islands put together. Oahu is great to visit - once!
Wyndham Waikiki Beach Walk is newer and right in the middle of the action - but not ocean front.
The Big Island (Hawaii) - The Big Island is bigger than all the islands put together. It has a lot to see, including the active volcano. Because it is the newest island, most of the shoreline and island is still rocky lava flow, and there aren't many timeshares on the beach. It has great snorkeling. It involves a lot of driving to do it proper. *Had a Dengue Fever outbreak in the last year. OK now - but check on it before you reserve.
I like the Wyndham Kona Hawaiian Resort.
Maui - Maui has the most demand for exchanges and seems to be the "in" place to go (at least right now.) It has a nice blend of natural and touristy things to do, and the very popular Ka'anapali Beach Area - a huge resort area. Pros - great beaches. Cons - it's starting to get too crowded for us. A big plus is that you can visit 2 nearby islands by ferry or a short flight - Lanai, and Molokai.
Don't have a Wyndham Mauai recommendation.
Kauai - Our favorite island. Kauai is the oldest island so the lava has had the most time to break down and it has beautiful sandy beaches and lush tropical jungles. Kauai is very rural - small towns and not much in the way of fancy shopping and night clubs. It's strength is great outdoor activities like snorkeling, hiking, 4WD, and ATV trips. It's pretty laid back, but does have a mall, major grocery stores, and some good restaurants.
I like the ocean front units at Wyndham Kauai Beach Villas.