Mr. Emmerick talks about this some in his book, Here, There, and Everywhere. The mono mixes were done first, and had the most effort and finesse employed. It was generally assumed that the vast majority of listeners would be doing so in Mono. The stereo mixes were done as somewhat of an afterthought. I am generalizing a bit, but Mr. Emmerick certainly considers the mono versions to be the better mixes. When I got the collection for my wife last Christmas, I chose the stereo versions in the USB package. No regrets.