Ben, thank you for your clear answer.

I'm aware that mixte's are less rigid than men's road frames but I guess they are much better than the lady's frames whose top tube ends in seat tube. With a less rigid frame, she will loose some 5% (or 10% I don't know) more power against a more rigid frame BUT I'm afraid she'll loose some 100% power if I buy her a road frame and she never rides it. Wives can be strange sometimes and mine saw a folding bike yesterday and said "this one looks so comfortable". An upright sitting position and enough space (absence of a high top tube) when she decides it is better to jump off the bike instead of breaking makes her feel more comfortable.

Being heavier another thing ofcourse. A lighter bike is must have. Is it possible that #2 bike is under 10kgs ?