I have spent one week in both Maui and the big island on two separate occasions. I personally prefer the big island because there seemed to be fewer people (probably just more spread out) and the snorkelling/diving areas were amazing. I will second that trip to Captain cooks monument which is absolutely incredible snorkelling.
That being said, when I was in Maui I had the unbelievable luck to get to swim with a gigantic pod (or several pods maybe) of dolphins. We're talking at least a hundred of these guys (adults and juveniles) all swimming around you. That was at Honolua bay and for that reason alone Maui really stands out.
Anyway, between those two islands the differences aren't overwhelming. You'll have a great time wherever you decide to go. One thing to consider is inter-island flights which could allow you to visit a couple islands if you wanted to.
Personally preferred the big island to Oahu. Much less people and when not in Kona much less touristy. Lots of little empty or nearly empty beaches to enjoy for hours on end. We spent two days on one little out of the way black sand beach only seeing each other and about 10-15 big sea turtles that were sunbathing with us. and that was during the Christmas season.
And Volcano National Park has all sorts of excellent hiking as well as Kiluea. Though we managed to go during the one week in 5 years that you couldn't see the lava actually entering the ocean...just the steam. Still an impressive park IMO. Loved every minute of the park and big island.
There was also a vineyard right near the big park which had some decent wines and what not. Some a bit unusual. Atleast to me at the time.
the one thing to keep in mind....get a rental car that corners and handles well. It will make driving around the islands much more pleasant.
Pearl Harbor overated?? Holy crap.. I guess in a way the touristy stuff there is which is why the're renovating the gift shops and whatnot but the location itself is pretty extraordinary. Just imagining all the chaos that would be going on back in 1941 put me in awe, seeing the old watch tower.
would be more than happy to quote a price for you on your trip.
dont forget about the CP discount.
give me a shout.. toll free at 1-888-723-4478 or email at axel@yellowheadtravel.com
Back in 87 I went to Mauii with my family and close family friends for 2 weeks around Christmas time. I know my dad wished that we would've done a day trip to another Island. Unfortunately I was too young to truly appreciate Hawaii, but it was a different story when I went with my wife and best friends in 2004. I think had we stayed longer than 1 week we would've done an inter-island trip.