Really just comes down to personal preference and specific eyes. Glass is so subjective that it’s really kinda pointless to discuss.
The MK5 that I spent time with was a 3.6-18x44 model. I feel that’s important to note as there has been some question as to whether they are the same optically as the 5-25 model (they are a good bit shorter and have a smaller objective lens). So maybe some of my opinion is skewed by the fact that I used the smaller model.
Regardless, the MK5 exhibited poor CA control whereas the MK8 did not. In high contrast conditions (bright targets against darker backgrounds), there was a discernible magenta hue/glow around the targets looking through the MK5... while some will argue how much of a difference CA really makes, it’s annoying to look at if nothing else.
Clarity and resolution wise I felt the MK8 was also considerably better as the MK5 had a tendency to wash out some of the contrast in bright conditions. This made target acquisition more difficult when fighting the sun, shadows, and mirage. I also found the focus/parallax settings much easier to set on the 8. Eye box and relief were about the same for me as I did not notice any real difference getting behind each scope. Ergonomics/controls/layout I won’t comment on as again, it’s very subjective.
Turrets and tracking I would call a wash as both optics performed very well. I did prefer the zero stop detent in the MK5 compared to the M5C2 turrets, but both had precise clicks devoid of any slop. Both were tactile and precise with good feel. Tracking-wise, I trusted both to do exactly as I told them to.
So while I am not saying the MK5 was “bad” by any means, it was a markedly lesser scope than the MK8 in my hands. Again, I think a lot of it comes down to preference, eyesight, etc. But with the price discrepancy between the two (really lack thereof based on what MK8s sell for on the secondary market), I believe the better value is the MK8.
