You won't be able to solder to stainless steel where as you will (might) be able to solder to silver.
Silver thread will have a lower resistance but as you say will tarnish. However, that only affects the joints and if you solder or use proper crimped connections that should not be an issue. Also silver is more ductile so less likely to snap but as you say it has a much lower tensile strength so it won't resist stretching as much as steel.