My wife & I own a timeshare on Maui, so we go almost every year. We've been to O'ahu, and while it's still tropical and nice in places, Honolulu is still just a big city...kind of like a tropical San Francicso.
As has been mentioned many times, the Big Island (Hawaii) is less developed in many areas, and the west side is pretty desolate with all the lava fields. The Kona coast can be an absolute traffic jam some times of the year however. (Too many people, not enough roads)
Hilo is ok, (still trying to figure out how we can move there) but not a lot of touristy things to do IMO.
Maui pretty much has it all. It has one "city" (Kahalui, where the airport is) that has Target, Costco, etc. where you can stock up on whatever you're going to need for the visit, yet once you leave there, it's more or less all country setting, except for the occasional small town...and Kihei, which I don't like.
The "local surfers" (even though most aren't locals, they just came to surf and never left) can be rude as hell to newcomers, there are a lot of drugs (they're everywhere, but seem to be worse there than most of the island) and because of that element, Kihei has more of a crime problem than the rest of the island. (except Kahalui)
Now anyone living there may disagree with my assessment of the Kihei area, and they will probably be more right than I am, I can only give my impressions as a tourist in the area. The snorkeling off the Kihei coast can be spectacular however, (lots of fish to see) but like ALL the beaches, don't leave anything of value in your car...odds are not good for it being there when you return.
Maui is pretty laid back, but is pretty heavily developed, so at certain times of the week, there may be quite a bit of traffic on the roadways. DO NOT get in a hurry. MOST of the roads on the island have a 30-40 mph speed limit, and the Maui County cops really enhance the revenue on speeders. Be aware, MANY do not drive cop cars, but drive their own personal vehicles...so you COULD get pulled over by any make/model of car/truck with a flashing blue light.
Someone asked about island hopping...Yes, it can be done, but with all the hassles at the airports, you basically lose a full day every time you change from one island to another.