The lagoon views are good if you want to be able to see the Electric Water Pageant or MK fireworks from your room. This is great if you have kids who need to be in their pj's by firework time. Some (but not all) rooms have a view of the top of the castle. Keep in mind that the castle is quite a distance away. Some are disappointed that the castle view is not better. I think it is still prety nice. You can get the exact same views from the Volcano Pool or the beach.
The garden views can be of the pool, the marina, shrubbery, flowers, or the monorail. We once had a garden view on the 3rd floor of Aotearoa that overlooked a service area. Not very nice, but still o.k. But back then the rate structure was for garden/parking view, garden/garden view, pool view, marina view, and lagoon view. I recall that we decided to be frugal on that trip and booked garden/parking view.
Have a great trip!