You do not need to choose between using a medium or a media query for a target. They can be used alongside one another when you generate output.
Example You have custom medium called "OnlineBlue," which you associate with one of your targets to show content with a blue theme on some styles. But in addition to that medium, you also make some edits to the tablet and mobile media queries in your stylesheet so that content is adjusted for smaller devices.
When you view the output on a large monitor, the content will look a certain way that makes sense on a big screen. When you view it on a tablet, the content might shift a bit so that it looks better on that smaller screen. And when you view it on an iPhone, the content might shift again to account for that device. But in all three cases, the content is still adhering to the blue theme from the OnlineBlue medium.
