In city traffic, is sitting more upright safer than more aero positions?
Being seen - the higher you are, the more likely you will be seen over a roof top. Clearly some cars are too high or too low for it to make a difference, but the odd car is at a height the difference might be significant. 

Seeing - in an aero position, its harder to see as much as in an upright. It can be done by actively looking around, but its harder - you have to force yourself to do it. 

Speed/efficeny - when I go into the aero position, its because I want to go faster or ride more efficiently. My concentration moves towards my riding technique than observing the road. However, it is entirely possible to sit upright and not concentrate on the road (e.g. Focus on music, sending text messages, last nights hot date....etc), so I believe upright is safer with the proviso you are concentrating the same amount on riding. 

Manoeuvrability - when on aeros your ability to change speed and directions is restricted. If aero position did not compromise agility, XC MTBers would have drop bars. 

So overall, I would suggest that Aero position is less safe than upright. Quantifying how much difference it made would be harder...