But they are turns of a circle of a given radius.
So as long as everything conforms to that coordinate system, we're groovy?
Radians (circle fractions) are generally preferred because we can compare, e.g. two planetary orbits, conveniently, no?
Or did I miss something?
It's the other way around actually. A turn is a turn, no matter the radius of the circle. Radians are the length of the line you need to draw a fraction of a circle with a radius of 1.
(In the end, both are just ways to describe angles and thus independent of any radii... the only effective difference between them is a constant factor of tau or two pi.)
No, a revolution is a revolution. If you turn a 1 m wheel by one revolution and a 2 m wheel also by one revolution, both will have turned one revolution, or 2 pi radians. If you roll both of them 2 pi m forward on the ground, one will have turned one revolution, or 2 pi radians, and the other will have turned half a revolution, or pi radians.