So if we're measuring ecological efficiency of Bitcoin, it'd be something like cents or watt-hours or grams CO2 per financial transaction. I've never seen a rigorous comparison, but I wouldn't be at all surprised if Visa and Paypal are more ecologically efficient than Bitcoin.
We store it in vaults...
http://geology.com/minerals/gold/uses-of-gold.shtml
I suspect that even a lot of the "investment" uses of gold are much more about human emotional needs. I helped someone sort out the house of a deceased elderly relative. There was quite a bit of stuff purchased and stored away: coins, silver, gold. A number were of the Franklin Mint variety, where they wildly overprice the metal:
http://coins.thefuntimesguide.com/2008/11/franklin_mint_coin...
I'm sure the guy was happy with his purchase, but as investments go it was a terrible idea. He would have been much better off putting his money in index funds.
In contrast when gold mining gets more efficient, either we expend less energy, or we produce more industrially-useful gold. Admittedly in the limit where people are just buying gold and putting it on the shelf I guess the same considerations as bitcoin apply? Hmm.
Gold mining and refining can be nasty stuff.