They can be made to be (much) better at tracking the terrain, give a more usable rising rate than air, can be made to damp more effectively than air, don't alter their damping charactersics as they heat up, etc, etc.
By "tracking the terrain" I assume you refer to the way the suspension makes the wheel track the undulations in terrain. If so, then there's no difference between the tracking of an air suspension over coil spring. The reaction speed is dictated by the damper and since both systems can have the damper adjusted in such a way that the wheel bounces rather than tracks, and both systems can have the damper matched to the track, there is no difference.
Damping has NOTHING to do with what type of spring the fork employs. In fact, the spring and damper is separate - in different fork legs, in many cases. They are independent. I cannot see how one can damp more effectively than the other.
As for heat - I'm yet to see a bicycle fork beyond ambient temperature, no matter what run it had just completed.
Etc etc etc???
The downside is they weigh more and can be rattly. This is why compressed air is virtually unknown as a suspension medium on motorcycles, where another 200 grams is not a problem to carry.
I'm yet to come across a rattling spring in a properly set up coil fork. The spring is always under compression and heavily greased. They don't rattle. Yet, I'm sure, somewhere on the internet someone will have a rattling fork.
Sprung forks can also be dreadful too, but going by their technical characteristics alone a coil spring is a more effective suspension medium than air for performance, and a top flight spring will perform better than a similarly top spec air system.
We are still stuck at "more effective".
What technical characteristics and what do you mean by more effective.
How do you define performance in a spring and how do you distinguish performance between air and coil?
Conversely, cheap air is liable to be better than a cheap sprung fork. At the lower end of the market air is probably a better bet, but if you're after sheer suspension performance and you're not afraid to pay through the nose, the few remaining top drawer sprung forks are still king.
I'm not so sure about that. Air suspension is expensive to make. You need pistons with close tolerances, seals and air valves, whereas spring forks don't need airtight chambers or valves or seals. I think it is the inverse. At the higher end of the market, air is king. The market is of course not homogeneous - there are different types of forks for different applications. At the downhill end it is coil dominated, at the marathon end, it is air dominated.
