Ag vs Cu: look it up! The conductivity of silver is only 5% higher than copper. In normal conditions, Cu oxidizes, forming a copper oxide semiconductor, but Ag reacts with sulfur in the atmosphere to form the black tarnish you see on silverware. The advantage of gold is that the surface will remain untarnished in normal conditions.
It depends on the trace gases present, but Silver generally does not oxidize as fast as copper. If you have a natural gas heating system, then you will have much higher trace compounds that will react with silver and cause it to oxidize at an accelerated rate.
I have many silver plated RF connectors that still look white and (mostly) oxide free that are well over 10 years old. On the other hand, I have bought random lots of vintage RF connectors from eBay that had some silver connectors that were black as sin (though they cleaned up nicely using the aluminum foil and sodium bicarbonate trick). I also have some stripped copper wire that has turned brown in just over a week (which can be removed with citric or ascorbic acid when tin or zinc is not present).
As for earlier comments about direct gold plating: Gold will readily diffuse into copper without a diffusion barrier like nickel present. However, if the plating is thick enough, then you can get away without using the diffusion barrier. Commercially speaking, this requires much more gold than manufacturers would like to use. With the exception of audio fools, you really only see this approach done on older test/calibration equipment. For example, this is the plating process HP used on the 419A DC Null Voltmeter's input terminals. Leeds and Northrup's binding posts (for bare copper wire connections) also use gold flash directly on copper. They also chose to save costs with a brass nut since the copper wire would directly touch the gold-on-copper terminal.
A diffusion barrier is really just a way to cut costs (and use a thinner plating). It is not a set-in-stone requirement.
Personally, I hate cleaning bare copper. So I tend to avoid using it outside of critical short term applications. Unfortunately, most modern gold platings (i.e., economically thin platings) aren't that great either. I've seen 40 micron or less platings of gold literally peel off due to holes in the gold surface coat that allow chemical attack of the underlying nickel to take place. Older stuff that had over 40 micron coatings seem to last forever under minimal connect/disconnect cycles. So there is some optimal thickness here.
From what I've seen firsthand, the "heavy" gold flash platings (usually having a dull sheen) have the highest wear characteristic since the plating is so thick. Thin platings of gold in high friction environments (i.e., a 4mm banana plug or cable) do not last long and the nickel under coat becomes visible after a while.
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