The the big difference between an Obsession and Starmaster is the Starmaster's removeable mirror cell. I believe the Starmaster has the truss tubes held together in pairs. I believe the surface of Starmaster alt bearings is a little different.
Obsessions have a have mounting points for their truss tubes on the outside of the mirror box.
They move very well and some people feel too well, to easily. Both brands are well made. You may prefer the look of one over another. A scope that is easy to load and unload from your vehicle. It should be easy to set up.