Personally, IMHO, I'd book the SPG nights at the Princeville on Kaua'i (or the Westin Maui) and pay for the next two. Then, book the HH points at the Hilton Hawaiian Village, then pay for the next two nights. This way, I wouldn't have the hassle of having to switch hotels mid-trip and it'd probably be easier on both you and your girlfriend.

As for the more romantic island... I'd say Maui is just a tab bit more romantic, since there's a bit more to do and it isn't as laid back as Kaua'i. Though, there are some great perks with Kaua'i, namely, it's isolation and small population. FlyinHawaiian has accurately described both above. Let us know what you pick!

aloha