Some people feel that bottom mounts also provides easier access for servicing the compressor.
Because the compressor must vent through the bottom of the unit the compressors can burn out faster. Top mount units give you more cubic feet of storage. All components of the top mounted refrigeration system are outside of the food zone so that the full interior is usable for storage. Top mount units run cooler than bottom mounts because there is not venting problems on top of the unit. They will also keep your kitchen cooler because the warm air is kept up and out of the work area.