It totally depends on if you want the top margin to always be 20px or if you want it to be 20% of the screen or containing div size.
To throw something else into the mix, you could also use ems (based on your 20px, that would be 1.25ems).
So basically do you want your top margin to be fixed or fluid / responsive (either way, use margin-top, not just top as that is for use with position: relative or position: absolute).
