ATX Power Supply Pin Material? (gold or tin)
Does the connector pin material matter (tin vs. gold)?

They need to be matched, if you are worried about 10mΩ's of resistance, then use gold
for most digital electronics this won't matter. Even with 2A of switching current (which would be worst case) this would only be in the 10mV range which wouldn't be a problem for digital noise.

If so, which should I get for interfacing with a standard
mid-to-high-end ATX power supply?

Go with gold to gold or tin to tin, otherwise you could get corrosion in humid environments.