On the other hand, you may want the cooling properties of having your midriff open for ventilation. We're all friends here. Either way, the choice should ultimately be decided by the race, the conditions, and the desire to look your best without sacrificing function no matter what you wear. There is a simple formula to figure this out. Take the wind drag X variable, multiply it by the AB coefficient of the tubing material, square to the fourth power of Powerbar, add your percentage strength of Mark Allen's power output during the 1992 Ironman, and then throw it all out the window and find the bike that is the most comfortable for you.
Comfort is the key.
Yes, a light bike will make you go faster and so will aerodynamics. However, in the end, it is the engine driving the bike that makes the biggest difference and making that engine comfortable will not only make you faster on the bike but save you more energy for the run. If you have to sacrifice a couple ounces because a slightly heavier bike is more comfortable, take the heavier bike. You can always loose weight by switching to lighter parts later on. Next, have a knowledgeable bike mechanic fit you on your comfortable bike.