From experience (my car prior to my GTI) I would not get an ex-demo. Probably given the beans from 0 miles and not warmed up and cooled down properly. My old Astra VXR should have produced 237BHP standard but was 20BHP down on that, more than likely due to being a 5000 mile ex demo.
Also, ordering new means you get exactly the spec you want, not what someone thinks you want.