Obviously, it's easier to compare if they are all relative to 100g, but arguably per-serving is more useful when making eating decisions. When I'm buying a food, I'm not trying to do science; I'm trying to figure out how much of a particular nutrient I'm personally going to get when I eat that item.
For example, if I have soup, there's no way I'll have as little as 100g of it. If I have tea, there's no way I'm going to have as much as 100g of tea leaves. The mass of a serving could be 1 or even 2 orders of magnitude different depending on the type of food.
Per-serving isn't standard between items though. The FDA requirements are broad enough that companies can make up their own serving sizes, and one company could consider a 12oz can one serving, and the other two.
If the two-serving brand is also advertised as low sodium and consumers don't notice that the serving size is half as big, that's gonna seem to be much lower sodium than it really is.
It can certainly go either way, but space and simplicity would be the arguments against including both. On a small label, you might need to use a smaller font. Even if you have enough space, a more minimalist approach can be quicker to read (provided that it contains the information you actually want).
Obviously, it's easier to compare if they are all relative to 100g, but arguably per-serving is more useful when making eating decisions. When I'm buying a food, I'm not trying to do science; I'm trying to figure out how much of a particular nutrient I'm personally going to get when I eat that item.
For example, if I have soup, there's no way I'll have as little as 100g of it. If I have tea, there's no way I'm going to have as much as 100g of tea leaves. The mass of a serving could be 1 or even 2 orders of magnitude different depending on the type of food.
Serving sizes are required by law to be "based on amounts of foods and beverages that people are actually eating" (see https://www.fda.gov/Food/GuidanceRegulation/GuidanceDocument... ).