I have a Giant Defy Advanced and a Windsor Tourist. The Windsor is about 10 lbs heavier. I'm about 4 mph faster on the Giant on a mostly flat course. Did I mention that the Giant has aerobars?
I also had a Bianchi hybrid (thankfully without aerobars) and compared it on the same course as the Windsor. The Windsor is a couple of pounds heavier than the hybrid, but I'm about 1-2 mph faster on the Windsor. The Bianchi hybrid had me sitting much more erect.
Looks like on a flat course, aerodynamics makes a bigger difference.