I think both are excellent choices for an anniversary celebration. They are definitely romantic. I'd probably consider location and theme to be my deciding factors. AKL has a more remote feel and you'd need to rely on Disney transportation or your own car to travel outside that area. They seem to have a nice variety of restaurants available right there though, with fast food counter service, Boma and the more upscale Jiko's. The low lighting, dark wood and overall feel of the resort is nice, not to mention looking out your balcony and seeing all those animals. The GF is also on the remote side, although you are within a monorail ride to 2 other resorts for food and the MK. The lighting and theme is much brighter here, but still soft and romantic. The garden views are just as lovely here as the lagoon views. Don't worry about overlooking a parking lot, or roof top here, it won't happen. You also have the beach here which gives you some more recreation options. You might also want to consider the Boardwalk area if you want a more lively locale. The BC is a casual theme, while the YC has a bit more of an upscale feel to it. The BWI will put you right in the middle of numerous restaurants, shops and entertainment. The proximity to both Epcot and MGM are great. We stayed at the YC this past December and with our park hoppers, we would hop into Epcot just to grab some lunch or a pastry from France. I think it was all of a 5 or 6 minute walk to the back entrance.
Picking a resort is never easy. I'd just make a list of the things you like about each one and go from there.
Tammi