Will you be accessing your unit nights, weekends or on holidays? Do you prefer a unit that you can access 24/7/365?
Aside from a good, strong lock are you looking for a place with fenced perimeter? Video surveillance?
Climate controlled storage is a great way to protect your belongings from harsh temperatures, humidity and pests.
Does the facility have a regular cleaning schedule and maintenance for each empty unit?
Drive up access (which makes loading and unloading a breeze)?