This question is asked a lot. So there's lots of old threads covering this topic. I've added some links below but there are plenty more.
The Big island will be easier to book than Oahu during peak season (whenever the kids are out of school) due to the competition. So if you want Oahu, it's best to book a room their first then book your Big Island stay the week before or the week after.
On Oahu, you'll want to stay at the Hilton Hawaiian Village (HHV) resort.
On the Big Island, you'll want to stay at Kings Land or Kohala Suites.
NOTE: The three bedroom at HHV are harder to book than the two bedrooms on both islands.
Location
OAHU - The HGVC resorts are located on Waikiki in Honolulu
BIG ISLAND – The HGVC resorts are located in the Waikoloa Beach Resort area.
There are several restaurants and two shopping centers within Waikoloa Beach Resort -
http://www.waikoloabeachresort.com/
Hotel Property Integration
OAHU – 4 out of the 5 HGVC resorts (Lagoon, Kalia, Grand Waikikian and Grand Islander) are located within the Hilton’s 22 acre hotel property (Hilton Hawaiian Village). As a result, HGVC guests have access to all of the hotel amenities (however at hotel prices).
The Hokulani property is not integrated with the Hilton Hawaiian Village. It's a standalone boutique hotel in the heart of Waikiki near the main shopping strip.
BIG ISLAND – None of the existing Club resorts (Bay Club, Kohala Suites & Kings’ Land) are located within the hotel property (Hilton Waikoloa Village) however all HGVC guests do have access to the Hilton hotel amenities. Waikoloa Suite and Kings’ Land guests have free access to the hotel pool. Bay Club guests must pay for hotel pool access. There is an internal HGVC resort shuttle that stops at the Hilton property and two local shopping malls.
Beachfront
OAHU - Hilton Hawaiian Village is a beachfront property. Many of the HGVC rooms on the hotel property offer great ocean views.
BIG ISLAND – While the Hilton Waikoloa Village is an oceanfront property, it does not have a swimmable beach (extremely rocky). None of the HGVC resorts are oceanfront. Some have a glimpse of the ocean. Here’s a map of the Waikoloa Resort area that shows the location of each of the HGVC resorts -
http://www.waikoloabeachresort.com/files/map2012.pdf
Pools/Lagoons
OAHU - Truth be told... I don't find the pools at HHV inviting. They've always seemed too small and cramped for the size of the resort. But the lagoon is a fairly large size.
BIG ISLAND – I can only speak about the pools at Kings’ Land and the Hilton Waikoloa Village. They are both inviting and relaxing. We spent a day at each pool location during our 2013 stay in the 1st week in June and truly enjoyed our experience. See HWV Lagoon and Pool photos -
http://www.tugbbs.com/forums/showthread.php?t=194066
Big Island
Interested in opinions on the 3 Big Island resorts (2016) -
http://www.tugbbs.com/forums/showthread.php?t=240857
Kings Land 2br (2012) -
http://www.tugbbs.com/forums/showthread.php?t=173030
Phase 3 at Kings Land (2016) -
http://www.tugbbs.com/forums/showthread.php?t=238439
Why go to HGVC on the big island at all? (2015) -
http://www.tugbbs.com/forums/showthread.php?t=236611
Oahu
For those who are not familar with the Hilton Hawaiian Village(2013) -
http://www.tugbbs.com/forums/showpost.php?p=1563081&postcount=31
Oahu HGVCs - Help me decide which one to stay (2013) -
http://www.tugbbs.com/forums/showthread.php?t=194183
What would you recommend in Honolulu? (2012) -
http://www.tugbbs.com/forums/showthread.php?t=163139