The difference, according to DOJ’s anti-trust division, is whether these firms are competitors.
Whether there is a difference is the test.
Whether harm will be done is also a test and the DOJ looks at the utility of the merger as well.
Measuring the utility, however, is complicated by philosophical argumentation, and these arguments tend to have the color and shape of Positivism, based on the “natural” order of things.
Positivists say that philosophical arguments are too ambiguous to yield accurate measures. To really know the difference, don’t use philosophical critiques. Instead, use empirical measures.
According to capitalists, income is the empirical measure of success; and capitalists spend huge amounts of money (income, “utilized” in the form of capital) to yield success.
Philosophically, the greatest good (the utility) approves of whatever naturally yields economic success. According to capitalists, the best way to produce it is to resist inflation (rising wages and salaries) and support pricing power (yielding the accumulation of capital that buys the measure of success).
(Rising wages and salaries also produce capital. The difference is who owns it and determines its useful value or measurable currency in the name of efficiency, which can be described as an efficient cause or purpose.)
Whatever just happens by natural design is, by default, justice. (There is observable symmetry of charge and parity under transformation, as physicists describe a natural law, having no difference — unchanging — no matter what.) The unchanging (absolute) measure is naturally positive and, according to capitalists, the DOJ is wise to approve of it (no matter what).