As a user of Poser/ Octane and DS Iray I can say they all create stunningly realistic renders... with software know how
That said... Octane requires you to know a wealth of advanced material/ lighting settings close to that of a photographer, in order to produce great results. Though the Octane plug-in for Poser greatly simplifies this process. You would still need to make adjustments.
DS Iray is free and fairly easy to use by comparison. That's if you don't mind using DS's rather clunky and awkward UI, in my opinion. Though it is a great entry point to 3D rendering if you are a beginner
Poser/ Octane (Standalone) and Poser to Octane plug-in don't come cheap. If you are dedicated to the pursuit of fine rendering and are willing to know your craft, then Poser would be the weapon of choice. If you are a hobbyist and want to try your hand at 3D then obviously DS Iray is a no brainer