silver vs gold is more about the variable silver/gold ratio - currently silver is undervalued compared to gold, but this is more of a current-day 'portfolio' trading concern than a 'situation X' trading for survival supplies iyswim.
The advantage of coins is you can 95% verify their authenticity by checking their weight & measurement against the official figures. Gold is so heavy there are few materials that could be used in its place. You either have a slightly lower carat or something from the platinum family (also valuable). Theoretically it could be something like plated tungsten, but this is extremely unlikely.
The disadvantage with (gold) coins for emergency trades is their just too big & the tiniest ones are sold at a massive premium. From this POV small items of hallmarked jewellry would be a better choice - and draw less attention to you as someone worth attacking. A few grams of '375' (9k) gold as earrings or wedding bands can be had for about £10.