I love Oahu personally, and all the pics of seen of Maui look amazingly overated. If I were to go somewhere romantic i'd go to Kauai. The Garden Island.
Really though, I can't see you not liking any of the Islands but if you want a combo of metropolitan, beaches, lush greenery Oahu has it all. Its the only Island i've visted to date, my bro is getting married next month in Maui so i'll have to get his input on the island but from the Google info I got, it looks boring.
On Oahu, make sure you go up to the Nu'uanu Pali Lookout. Take the Pali Highway north. The best view i've ever seen.
Here's downtown Honolulu
Here's an amazing pic I took on the windward (North) shore
You really can't go wrong with Hawaii though, any of the islands. Just ask Kipper who's been God since coming back from there. lol
I went on a Air Canada Vactions thing. Stayed at the Miramar Waikiki lots of cool stuff to do around there.
Here's one of the Eastern side of the Island past Diamond head