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As far as I know PoW leads to more decentralization (especially in combination with ASIC resistance) and therefore a more secure network. Energy efficiency comes through advancements in chip manufacturing.
Improved energy efficiency only results in a higher hashrate with the same amount of energy consumed. Assuming electricity costs dominate the mining expenses, in an efficient system the value of consumed energy should equal the value of mined coins. If it's significantly less, more miners will join until everything is balanced again.
On the flip side this means it's very easy to calculate the running cost of a financial system based on XMR: it's roughly 0.8% of the supply per year, slightly decreasing in the future thanks to the tail emission.
If you use the same metric to determine a cost for the current central banking system, which targets a real inflation of usually 2%, but it's more like a 4%+ cost of living increase for necessities, meaning using monero is AT LEAST 5 times as efficient. However this is ignoring the fees banks demand at every opportunity, so I'd estimate monero is about 10x as efficient.
Fair point. The market will find a equilibrium between electricity costs and mining reward. Also compared to the current banking system miners will probably move their business mostly to locations where energy is cheap and highly available.