The usual way, i.e. "efficiency=performance/power"? Here, I did the math for you. Numbers from whattomine.com:
https://i.imgur.com/2u2HOw7.png
https://filebin.net/fr1sqfwtvulkoynu/gpu_efficiency.ods
As you can see, apart from Vega being a very efficient Cryptonight miner, and Polaris being an acceptably efficient Ethash (Ethereum) miner, AMD cards have absolutely garbage efficiency, they use just tons of power for their performance. And they are one-trick ponies: there are only one or two decent algos for AMD cards.
The 1080 and 1080 Ti specifically have problems with a few algos due to their GDDR5X (which results in half of the memory bandwidth being wasted) but on the whole NVIDIA cards are extremely efficient miners across a variety of algorithms, and usually keep up with AMD on efficiency even on Ethash.
The 1070 Ti, in particular, is the reigning champion of efficiency. It's basically a 1080 with GDDR5 instead of 5X, which makes it the most efficient card in most algos, and its worst-case is "only" being as efficient as a 580 at Ethash.
Again: people prefer AMD cards because they ROI quicker during the booms. The half of this that efficiency obscures is power: AMD cards have higher TDPs and are capable of pumping out a lot more watts per dollar (RIP Planet Earth). But when profits are down and efficiency matters (or for those with expensive electricity), NVIDIA is better, and you also have that flexibility to move across algorithms if there is a hot new currency.
There is also a pretty substantial cargo-cult with regards to mining: people use AMD because that's what they see other people using, and they don't bother to do the math. But NVIDIA has the same efficiency advantage in compute as they do in gaming: 1.5-2x the performance-per-watt in most situations.
https://www.techpowerup.com/reviews/Sapphire/RX_580_Nitro_Pl...