If you've never rented a storage unit before, you might be a bit overwhelmed by the options. A lot of options are just a matter of personal preference, but not those that have to do with security! Make sure the self storage unit you choose offers the four following security features so that your items do not end up stolen or damaged.
A Secure Perimeter
You don't want just anyone to be able to wander onto the storage facility's property. Make sure there is some sort of barrier around the facility (usually, this is a fence) and that the perimeter is secure. If there are big holes in the fence, this is a sign that 1) the facility is not as secure as it could be, and 2) the owners are not as vigilant about maintenance as they could be and thus may not be very vigilant about security, either.
An Available Employee or Owner
You want to know that there is someone on-site that you can talk to if you have any concerns and so you can receive assistance in a timely manner. Having someone on-site will also deter thieves. Try to avoid renting a storage unit from a facility that does not maintain its office on the same property as the storage units.
Even if there is someone on-site during the day to watch over the units, the facility is likely left unmanned at night. A security camera system should be in place to ensure any legal activity that occurs after-hours, or out of the sight of the employees, is captured. The best security facilities have individual cameras outside each unit, but if you're not storing anything amazingly valuable and the facility is in a good part of town, cameras near the entrance and at regular intervals throughout the facility are likely sufficient.
A well-lit storage facility is less appealing to burglars than a dark one since they'll be wary of being seen. Good lighting also helps ensure that any footage from a security camera comes out well. Make sure you specifically check the lighting near the unit you're planning on renting. You don't want to rent a unit in a dark spot.
If a storage unit has the four qualities listed above, you can count on it being generally secure. Above all else, make sure you rent a unit from a facility where the staff are friendly and willing to talk with you, so you can be confident that if there is ever a problem, you will have their support in dealing with it.