Yes, the supertele will get you noticed! I doubt there'd be much real difference between the 500 II and 600 II in terms of getting noticed. Having said that, with the hood in place, the 600/4 is the largest of the current Canon lenses (bigger than the 800/5.6).
Personally, I shoot a lot of birds so the 600mm is the better choice. Combined with the 1.4x III, the resulting IQ at 840mm f/5.6 is equivalent to the 800/5.6 lens, and while no shorter, the combo is a lot lighter.
And you can go back to just 600 with an extra stop of light, but not with the 800.