The appropriate comparison is Maui versus Oahu.
" What can we expect?" - You can expect a very different experience. If you are truly open to that, then you will have a great time. But if you expect Oahu to be exactly like Maui, then you may be disappointed.
The point is well-made above. You'd be choosing between Maui and Oahu. I recommend that you hop over to your local bookstore (or use Amazon) and pick up a travel guide or two. And you need good, real maps.
The Islands are very different in topography, development, history ... lotsa things. Population!
I think in the most general terms you could say you can expect a more "urban" experience. Would you be interested in things like seeing Iolani Palace, Bishop Museum, Doris Duke ShangriLa Estate, Honolulu Museum of Art, that kind of thing? Hiking DiamondHead. Snorkeling Hanuama Bay?