Both are used to compensate the owner for potential damage done to the vehicle.
The insurance excess concerns the damage for which the insurance has been subscribed, whereas the security deposit covers the damage that the insurance does not cover (i.e. broken dishes, missing fuel, ...etc)
Please refer to our insurance page in order to find out more about the amount of the excess depending on where you are travelling.
Please bear in mind that the security deposit can be used to cover the insurance excess in the event of an accident.