It probably comes down to two things.
1) Would you like a view of the monorail whizzing by (some kids love this)? If so, request Sago Cay.
2) What do you want to be closest to? Bus Stop/Gasparilla's - Sago Cay. Boat dock after long days in MK - Conch. Themed pool and main building - Big Pine. A little of everything/don't care - Boca Chica.
There is a slightly different garden view available in Conch and Sago Cay - that looks out to the marina inlet from the lagoon. It's still garden view category, but a little different viewing.
You're not going to be looking at a parking lot in any of the buildings. So there aren't really better/worse garden views. It's more about the location of the building, and even the location isn't a big deal because the resort buildings are not that spread out. 5 minute walk max from anywhere on the resort to any other point.