Why choose a larger TT frame?

Hi.
I'm in the process of buying a FELT DA (timetrial) and I don't know what size to choose.
I'm 192cm tall, and I'm not quite sure if I should choose the 56 or 58.
I don't know what the advantage of a 58 would be.

Due to the UCI rules there can't be more than 75cm (80cm for taller riders) from bottom bracket to the tip of the aerobars. Which means a larger frame wont do me much good.
The advantage of a smaller frame is the fact that the headtube is lower. The advantage of the larger frame, is the wider wheelbase - which effect that has... I don't know.
I'm assuming I would be able to raise the seatpost of the 56 frame to acommodate my height?
Unforutnately I haven't been able to find a dealer here in Denmark - so I can't try one out before buying. I hope you can help

To sum it up, why choose a larger frame when the length of the bike is limited by the UCI and a lower bike would make me more aerodynamic - and still allow me to have the same seatpost height?


Thanks.