Let's look some data:[edit: Soy Milk protein require 2.9x less water than Caw Milk protein, DIAAS adjusted: 1 / (0.28 * 1.25)]
PDCAAS [0]
Soy 0.92 - Beef at 0.94 (Beef 2% better)
DIAAS [1]
Soy 0.91 - Beef 1.116 - Whole milk 1.14 (Beef 23% better, Caw milk 25% better)
WATER footprint [2]
Soy Milk is 28% of caw milk (Soy 257% better)
Soy Burger is 28% of caw milk (Soy 1329% better)
CALORIC / PROTEIN fed population [3]
MAD (mean American diet) 341m - Plant-Based 531m (plant 56% better)
0: https://ourworldindata.org/land-use-diets1: https://en.wikipedia.org/wiki/Digestible_Indispensable_Amino... No data for Soy PDCAAS in that page and Beef has a different number as the first source. The difference isn't significative though.
2: > The water footprint of the soy milk product analysed in this study is 28% of the water footprint of the global average cow milk. The water footprint of the soy burger examined here is 7% of the water footprint of the average beef burger in the world.
https://www.waterfootprint.org/resources/Ercin-et-al-2012-Wa...
3: > [caloric and protein] At 3% in both metrics, beef is by far the least efficient [...] legume-dominated plant-based diets substitute beef with a dietary shift potential of ≈190 million [current US] individuals.
https://iopscience.iop.org/article/10.1088/1748-9326/11/10/1...
Obviously the production method of soy/burger/milk is significant, however others sources show rougtly the same order of magnitude: https://watercalculator.org/water-footprint-of-food-guide/
> depends on your body's protein needs.
For an higher protein/calorie ration, Quinoa is in range of soy' water footprint, and legume protein concentrates fit as well.