Thanks!
I think image quality is more about the user then the engine. Think of it like a photographer using a Nikon for some shots and a Canon for others.
Technically speaking, Corona is much much easier to use with much less input needed. Redshift has a steeper learning curve and a different philosophy on fine controls for all aspects. I also think that Corona is better intigrafed with the traditional Cinema workflow.
I still use Corona for my professional work. I added Redshift mainly for Volumetrics, which Corona is quite poor at. I now have clients using Redshift on our current project, hence the deeper dive. In learning it more, I've found that, for me, Redshift does bump and dispacment better, but it's not a show stopper for Corona by any means.
Redshift will only really pay off if you have a good GPU set up, but if you are happy with Corona, keep going. Once I committed to Corona and really learned it, I could focus more on the design and lighting, and less on "rendering". Corona remains my go to for most jobs.