From the outside looking in, I would rather have the MK57 compared to the AR57. The delaying action and upper receiver parts compatibility of the MK57 has a greater value to me than the AR57 upper. To me I see that Mk57 as being a good backpack gun candidate. Since it doesn't use the P90 mags a person isn't limited to a minimum hand guard length like what the AR57 uppers are using. Therefore, the overall length of the MK57 can be significantly shorter. Personally I do like the P90 magazines. When I worked at a range that had a rental MG, I just found that loading them became monotonous. I don't recall seeing a magazine related issue, so I am assuming they are reliable magazines. For the uses I would employ the MK57, using the Fn 57 mags would simplify storing spare magazines in a back pack.
One thing I would be interested in and perhaps Seb1 could elaborate on this is the at the ear noise of the Mk57 compared to the P90 and the AR57. I think we could all agree that a delayed action is quieter at the ear than a direct blowback when the ejection port is on the right side; ie a CZ scorpion vs a MP5. I would be curious to see how the bottom ejection of the P90 and AR57 compares to the MK57's delayed action but right side ejection.