For us, some of the primary differences were:
Waianae Building
- Easy access to parking garage which is right behind this building
- Fitness center and spa were in this building
- Quick access to Ulu Cafe, the Quick Service (lots of people order the pizzas) dining
- Close to the pools and jacuzzi that open the latest
Ewa Building
- Aunty's Beach House (for the kids) and the Community Room are on this side
- Ama Ama more upscale dining is right next to Aunty's
- Outdoor movies and the Luau are behind this building
- Sunset would be viewable from most of the rooms on this side
Here are some photos from Ewa side (16th floor) from our recent trip:
Here are some photos from Waianae side (9th floor) from a previous trip:
Make sure you request an ocean view room overlooking the pools, otherwise you might get a room on the outside. If you haven't seen it already, there's a great thread here with all the room views folks here have submitted:
https://www.disboards.com/threads/aulani-villas-views-updated-03-13-2017.3570961/
Best wishes to you on your trip!