I also think the Leupold Mark 5 has better image quality than the Mark 6 and, at a minimum, ties with the Mark 8. When the Mark 8 was first introduced, I spoke to one of the directors at Leupold and learned there was a single lens in the Mark 8 that cost Leupold $800 to purchase from the glass manufacturer. (Leupold makes everything in the Mark 5/6/8 scopes except the glass lenses. Those they design and have made by lens manufacturers.)
That single lens was expensive because it was made from a very uncommon type of glass that, at the time, only existed in small quantities. And was extremely difficult to produce. In the intervening 10 years, that glass type has become much more common and a lot less expensive to manufacture. That very same glass type is all through the Mark 5.