Obviously it is easier to get into a garden unit, the windows and doors can be reached from the ground. Also many doors for these are below grade and not visible from the street, so it would be better if the building had a common security door before going down to your unit. I lived in a building off the southport corridor in 2006, the garden unit was burglarised through the front door, which was hidden under the stairs to my first floor unit.
You also need to consider flooding. During heavy storms the last few years I know several garden unit owners who have flooded, ruining floors, walls and furniture and potentially leading to mold.