We spent two and a half weeks in Hawaii in September 2016 and it was great! Its considered the tail end of summer, but still a slower time of year with all schools newly back in session. We didn't find it particularly busy anywhere we went and the weather was beautiful. It was mid 80s and sunny each day, and we experienced no rain the whole 16 days we were there. We did note that Maui was particularly windy and Kauai was very humid, but we thought Oahu weather was perfect.
How many people in your travel party? It was just DH and I and we stayed in a studio. We thought it was tight just for two people with limited storage, so if you have anymore than that, I would book a 1BR. Its also nice to have laundry facilities in room so you can pack less - we used the communal washer and dryer (laundry tokens were free for those staying on points in a studio), but there were waits and a few machines were out of order. Its also nice having a full kitchen for anymore than 2 to help with your food budget. Its very recommended to rent a car while at Aulani, so many of us make grocery stops (even just for drinks and snacks, we don't cook) so having room to store these things is nice.
If you can swing 8 days, that's what I would do. As PPs mention, it will take a good day or two to adjust to the time change. I'd also recommend a good mix of onsite resort days and offsite island touring days, so having ample time to dedicate to each will make it worthwhile. As PP mentions, weekends are busy at the resort with local visitors, so we liked planning offsite touring days over Saturday and Sunday to avoid this. Traffic is also easier on the weekends when driving around.
