I understand. Truly. I went back and forth myself - because I wanted that Disney "feel". But if it is just you & David, what are you going to do most? Massages, rest, adult time. So probably spend more time in the adult clubs slow dancing, and in your room. And you still have the land portion (WDW before).
$500 is a lot of money you can put somewhere else. However, it is not a lot of money if it will make a difference in how much you enjoy your anniversary. If you switch to Carnival, save $500 and are disappointed that you aren't getting the Disney feel you were looking forward to, then it was a wasted savings. You know?
This is a celebration - you need to do whichever one will make you feel happiest.