I have a link from a 2018 study that shows the benefits of unions, but I wasn't able to find any studies that pit unionization vs min wage. I'm sorry I couldn't do better, but I'll share where I'm coming from.
Denmark, where fast-food workers make something like $20 per hour, doesn't have a minimum wage (as other posters have noted). I'll see if I can circle back to this later, but I'd guess that countries with strong union presences have higher rates of people who make more than whatever the minimum wage is for that country. Rather than a race to the bottom, and a constant need to redefine the bottom as it gets too low, you (collectively) have a seat at the table, where you can voice your wants, demands, and concerns.
Setting a minimum wage, especially in a country as large and diverse as the United States, is a pretty convoluted process. I know HN has some pretty large threads discussing the $15 min wage proposal from a few years back. A living wage is different in Seattle than it is in Boise, etc., etc..
It's convoluted, and it relies on legislation. A union says, "We're not doing shit unless we get X, Y, and Z." People hoping for a minimum wage increase beg people to consider the poor. So, setting aside the fact that one is arguing for middle-class (or upper-class) accommodations while the other is arguing for lower-class accommodations, one is arguing from a position of power while the other is asking for pity. One is saying, Your operation will grind to a halt if the compensation isn't adequate, and the other is saying, Please do the right thing, and give me enough money to live.
https://academic.oup.com/sf/article-abstract/97/2/495/502559...