I just found out that Mozjpeg uses arithmetic coding and Guetzli doesn't, so this wasn't a fair comparison. After repacking the Guetzli image using jpegtran the

I just found out that Mozjpeg uses arithmetic coding and Guetzli doesn't, so this wasn't a fair comparison. After repacking the Guetzli image using jpegtran the subjective quality at the same bitrate is so similar I can't pick a winner.