For astrophotography if you had to choose only 1 of these which one would it be? I can't afford a new full frame camera now, want to get into astrophotography. Also want a small travel camera anyway.
Or would you use the EOS-1DS and buy a fast lens for it?
The Canon EOS-1DS is an old school 17MP Full Frame. Canon's first real Pro camera. It should be a good Astronomy Camera and decent street camera.
I would start with the setups you have now. 17mm for Milky Way shots and the 200mm for shots of the Orion Nebula.