I'm just thinking out loud but I would think that a true U shaped bike would be lighter (given the same frame size) since you aren't installing that long top tube. This would be a plus, IMHO. There are some "female" varieties of bikes where the top tube is still there but slopes downward. This is what I would consider a true U shaped bike:
http://www.thebicycleescape.com/Imag...omPink_BIG.jpg