I have a Mk III but I’ve never heard its predecessors. I mean, it sounds really good to me and I think it’s worth spending the extra money for the latest model.
Really all generations on the Cronus are nice, but as you move up, you generally end up with a lower noise floor/background and improved phono stages and headphone amps. If you don't plan on using either of those (phonostage and headphone amp), then the Gen II tends to be the best buy on the used side in my opinion.
@goldprintaudio Is the phono stage better overall, or is it primarily the added features of MC support? I have a medium-output MC cartridge, which can run into an MM stage or an MC stage, and can be loaded anywhere from 47k to 200, so I feel like I could go either way.
MC capability was the main feature, but the stage was supposed to have been improved in general as well. With that said, I've never really done a back to back with a CMII versus CMIII only using the phonostage to see if you can tell a noticeable difference.