On my screen, I can see "halos" around the imageOptim compressed files. I choose to stick with JPEGmini because of the ease of workflow when integrated with Adobe Lightroom. I have an export preset that resizes and compresses with JPEGmini. With the free WordPress to Adobe Lightroom plugin, I have an export preset that resizes, compresses, and exports directly to my WordPress website.
I think that comparing the options for yourself is the best option.