Which is better for the pressure gauge movement, copper or stainless steel?

It depends on the application of the pressure gauge. There are many materials for the pressure gauge interface. But in addition to stainless steel, some pressure gauge interface materials are made of copper alloy, mainly bronze (copper-tin alloy) and brass (copper-zinc alloy), cupronickel (copper-nickel alloy). The bourdon tubes of ordinary pressure gauges are mostly made of copper alloy (alloy steel is used for high pressure), while the materials of the bourdon tubes of ammonia pressure gauges are all carbon steel (or stainless steel), and copper alloys are not allowed, because ammonia reacts with copper and will explode. Ordinary pressure gauges cannot be used for ammonia pressure measurement.