Riding down on tri bars on a very low time-trial bike is very very different from riding a sit-up city bike, to take two extremes. I would not like to ride the city bike down a mountain col, give me a race bike for that, please (and I'm talking from a position of safety here, not about riding faster).
If we're playing that game I think I'd prefer the upright city bike, assuming it was a decently made German-style one with proper brakes. The about-to-land-on-your-face factor of DF racing bikes is terrifying downhill, as are rigid bikes with skinny tyres. The more speed you can shed through aerodynamic drag the less braking you have to do.
Obviously a recumbent is even better. Something like a HPVelotechnik Speedmachine would probably be about right. No fear of road surfaces, all the braking you could wish for, and the ability to go round corners
[1] at speed, which the city bike may be lacking in.
OTOH, I know which I'd rather ride *up* the col...
If we assume poor road surfaces, then that has to be considered in bike design. Small wheels do not cope as well as large wheels when riding through deep potholes.
Quite. Small wheels are strong and light, but shit at potholes. Skinny tyres are a liability on bad surfaces. If the riding position involves either sitting bolt-upright on a saddle or lying in a recumbent seat, you benefit from rear suspension. If it involves bearing weight on your wrists, you benefit from front suspension. But then you're adding weight, and unless handled with tranquillity, suspension can waste energy at the drivetrain. How important is efficiency? This stuff is the bread and butter of bike specification, particularly recumbents, where people are less committed to a few standard designs.
[1] I can generally descend faster on my Streetmachine (an overengineered heavy tourer with wide tyres and suspension) than on my Baron (a skinny-tyred low-racer), simply because in the real world resilience to crappy road surfaces is usually more important than coefficient of drag. The exception is when it's not a straight line; the Streetmachine is designed for stability, and is rubbish at high-speed cornering.