We've stayed in both. Usually we go with the lagoon view if we can get a good discount.
Views from the Garden View Rooms are:
*Monorail
*Marina
*Courtyard Pool
*Garden Area
Views from the Lagoon View Rooms are:
*Seven Seas Lagoon/Magic Kingdom
*Seven Seas Lagoon/MK Resorts Area/Magic Kingdom
*Seven Seas Lagoon/Polynesian Resort
It's really a personal preference. Do you want to see the rest of the GF, or the lagoon from your room?