I decided to use only 'px' from now on, because that enables me to define all sizes myself. 'em' are being calculated based on the screen resolution and local system settings. Of course, I will need to implement an own conversion method for different devices when I don't use 'em', but I like to have control over how exactly different screens are being handled.

Though working with sizes in 'px' might not be the easiest way, it gives me the most possibilities. For example, I could make the sizes completely configurable in the SteemWorld Settings, which I anyway will need for my coming 'Theme Builder' ;)