Your body creates a majority of the drag, which is why aero bars are such a cheap source of speed. If you get around to trying the bars on each bike, you might do a comparative test on a short course - the position you end up in on a bike will be a larger factor than the aerodynamics of the bike, assuming similar wheels. So you may be faster on one bike simply because your body is more aero. Tight fitting clothing was another good recommendation - I see so many people with loose jerseys riding TTs. They are giving up handfuls of seconds to those with tight jerseys or skinsuits.