Well, it depends of course. 5µm Au plating is better than 1µm. And it also depends on the contatcs. A special double contact as you can find it in the S-relais from SDS (today Nais-Matsushita) having 5µm on each contact is comparable to 25µm on a standard contact. And 5µm gold plating in combination with a hermetic sealing is much better than 5µm without any sealing.
Also, life expectancy of gold plating highly depends on switched loads. With heavy load you can burn away all the gold with only one switching event, but with very light loads the gold plating can be alive for many years.


In any case, a hermetic sealing, maybe even an inert gas chamber, if possible, and as much gold on the contacts as possible.
It's often worth to contact the manufacturer of switch or relay and ask him whether he has a version with massive gold contacts. Often they do have such a version, but not necessarily being mentioned in their catalogue, which represents the standard program.

Kai