We use a two post style lift here in our shop, and there are pros and cons to both. I would say overall a two post is the way to go, but you really have to plan your job and set your vehicle up properly before you use it.
As far as worrying if someone will use your two post lift while your not around, PULL THE FUSE. That way you will not have to worry about your drunk uncle Larry having his 87 Buick falling off of the lift and possibley knocking some sense into him
