If you want your players to have a nice experience with no lag or anything to slow the servers, I would go for Hollow, because it would create fewer voxels and memory for both the game and studio, filled-in terrain would create more memory and voxels, causing low-end devices to lag or possibly crash.
My choice, Hollow.
Off-topic to the topic, but maybe some way to help in terrain building: Building Optimisation | Tips and tricks!