If inside doesn't have an ambulance, you need to call 911 first. Then, when 911 is called, and the "real" response is on the way, call the security and let them know 911 is on the way for a medical emergency. They can send something too, or not. But delaying an ambulance response to satisfy security's power trip is a bad decision.
Its clear from the this post that you've had absolutely no first aid training whatsoever. Even someone trained in basic first aid getting there a couple of seconds faster can save lives, security will call the emergency services and guide them to the correct location. If they are remotely competent they will do this faster then you can, whilst allowing you to provide first aid to the person.
If you have three other people with you that is the only time you should consider calling security and an ambulance, and you should call security as the first priority. (One person for first aid - priority one, one person to get better trained people, AEDs, etc and also allow the ambulance access to the site -- priority two, one person to call emergency services directly - probably unneeded but better safe then sorry)