IMO there is no "best". It comes down to what trade-offs you are willing to make in order to maximize achieving your objectives.
Personally, if I'm going to stick a long suppressor on the end of my rifle, than I may as well maximize the volume of that suppressor. I would personally take a Magnus over an Ultra 9 (haven't shot either, FYI). Part of that is increased suppression if I'm already going to put a long can on a rifle, the other part is that the Magnus has the hub option.
I'm already setup for the Q Plan B/Rearden MFG mounts on my rifles, and I like playing Lego. Not having hub capability precludes the Ultra-9 from my options (likewise the Dominus, which otherwise seems like an excellent all-around versatile can).
The Ultra 9 seems like an excellent precision rifle suppressor though.