JMHO, but if you will truly only be going ONCE to WDW, but are able to get to DL more frequently, I might wait a few years for WDW. I agree that some of the 'magic' will be gone for her, as far as believing in the characters, and princesses and all. But, at least in my experience, MK is by far the favorite for younger kids. While MGM, EPCOT, and AK have GREAT things for younger kids, they also have a LOT that could be experienced and appreciated at an older age. Since MK at DL and WDW are suppose to be quite similar, I would probably wait until she were 9 or so to do the once in a lifetime trip to WDW, so she could get the most out of all the parks. But I would take her back to DL one more time in the meantime to experience the magic of a young child at disney!
Again, JMHO!