My 2 cents ...
The "specs" should be part of the purchasers "requirements" not something we "evaluate" ...
"image" however subjective, must be part of the evaluation, because other than price, "image" (or perception of image) is the primary factor folks use to make a decision. And yes, since only one data point will be taken, the results will not be conclusive (IMHO), but at least the units will be compared in the same conditions, side by side, hopefully by the same set of evaluators. So the data itself will be part of a valid test ... its just scientifically I would not place a lot of weight on the interpretation of the data, because there will only be one data set collected on one night in one location.
So "image" could be totally subjective, or it could be broken down into two parts
(a) Detection Distance (and this not what the spec sheet says, this is what the observers say it is for them at that moment, with those settings)
(b) PID distance (again what the observers say ... PID is way tougher than detection, because ideally there would be critters out there to PID, and the critters might not show up
... but if there is a big field with cattle, that might work. I often see yotes around cattle and coons and deer also. So hopefully the critters will cooperate.
A night capable range finder will be required equipment so the distances for Detection and PID can be measured.
So anyway, my vote, is not to evaluate based on the hard data factors like magnification, weight, length, etc. those are facts and should be part of the purchasers requirements. Even "price" is part of the requirements, not an evaluation factor. The purchaser specifies his budget and that is that. It is not right or wrong or good or bad, it is part of the requirements.
The evaluation should be on the soft factors, specifically image. But also egonomics, user interface, those sorts of things.