Demonstrations are just that and are usually “prepared” and “controlled.” For purposes of even the most basic evaluation you have to simulate real conditions. You need to control the environment, not the salesperson. Remember, they are not going to highlight their weaknesses.
One thing to consider, if you are going to compare manufacturers, test them on an equal footing. The best way to accomplish this is use the same feeds from your cameras to the test equipment. If possible, try to conduct the tests simultaneously, as then you can make a true apples-to-apples comparison. Otherwise, there may be other mitigating factors that may skew the evaluation.