I've had all types of locks - double key, single key, allen wrench-lockable. I can't imagine that in a hostage situtation that the lock would make much difference. If the robber wants the door locked, he will command someone to lock it. In the end, the gun is what keeps hostages in and police out...not a locked door.
Personally, I like the double-keyed locks. Easy to use, secure, little kids can't fiddle with the locks, etc.