I'll be going to Hawaii in May with my girlfriend. I am flying into HNL and plan to spend 4 nights in Oahu. I have 3 days/4 nights to spend on another island, and I want to choose Kauai and Maui. Can someone compare the two islands in 2-3 sentences? Which of those two is more romantic?

My other question is are hotels in Waikiki cheaper than hotels in Maui/Kauai in general? I have 2 free nights via Starwood and 2 free nights via HHonors, and I'm trying to decide which island to book on, and which island I should pay out of my own pocket?