I didn't realize anyone really took the time to compare JUST these two fishes to each other nutritionally, until I did a Google search and indeed there were quite a few articles.
From this one:
https://healthyliving.azcentral.com/health-benefits-haddock-vs-tilapia-17015.html
Basics
Both types of fish have a mild, slightly sweet flavor and firm texture. Most tilapia is farmed in China, while haddock is harvested from the Atlantic Ocean. You don’t need to worry about mercury contamination in either one, according to the Natural Resources Defense Council. A 3-ounce portion of tilapia has 109 calories, compared to 76 calories in a serving of haddock. They’re both low in fat and high in protein, providing at least 30 percent of your daily allowance of protein in a 3-ounce serving.
Haddock Benefits
Haddock contains double the amount of vitamin B-6 in tilapia. You’ll get 0.28 milligrams of vitamin B-6, or 22 percent of your recommended daily allowance, from 3 ounces of cooked haddock. Vitamin B-6 activates more than 100 enzymes that metabolize nutrients into energy; it helps produce the neurotransmitter serotonin, which controls mood and regulates sleep and body temperature; and it may contribute to heart health because it lowers the levels of the amino acid homocysteine in your blood. High levels of homocysteine increase your risk for cardiovascular disease.
Tilapia Benefits
Tilapia provides almost four times more iron and six times more vitamin D. Iron deficiency is the leading cause of anemia in the United States, according to the Centers for Disease Control and Prevention. Lack of iron also weakens your immune system. Like iron, vitamin D helps support your immune system, but it’s especially vital for maintaining strong bones. Your body can’t absorb calcium properly unless you have enough vitamin D. A 3-ounce serving of tilapia contains 0.6 milligrams of iron, or 8 percent of men’s and 3 percent of women’s daily allowances. The same portion has 128 international units of vitamin D, which is 21 percent of your recommended daily allowance.