I would suggest that perhaps this is the wrong question. On the surface, it's pretty obvious that replacing four huge companies with three huge companies in any given industry will result in less competition overall within that industry.... The Justice Department's antitrust division has been exploring whether the deal would result in a major threat to competition. ...
Rather, the real question (and the reason why it's taking so long to come to a decision) revolves around whether or not both Sprint and T-Mobile are going to continue unhindered into the future without the merger. The international parent companies of both respective US based operations have grumbled about the US market and implied at different times that they might just pull out. If a failed merger attempt prompts one or the other to finally make good on that threat, than we're abruptly down to three (or two!) titans anyway, and each of those would then get a chance to vie for some portion of the displaced holdings of the defunct operation(s), including their customer base, antenna infrastructure, bandwidth licenses, etc. That kind of an ending could potentially enable the richest of those remaining to increase their advantage over the other carriers. In contrast, allowing the merger would theoretically push T-Mobile up to near size parity with the other two behemoths.
Bottom line: It's a very complicated gamble with a lot of moving parts, and there really is no way that the DOJ can know for sure which road will lead to the best overall outcome.