I deduce that this is totally hypothetical. Let's break it down. We can ignore the Stew-Mac brand name on the blocks; let's say we have generic radius sanding blocks. We can also ignore the dot inlays; they are easy to sand and can be replaced cheaply and easily if necessary; they have no bearing on my decision.
If the fretboard had binding, then I would rather flatten a 12 to a 16; that way if the slots became too shallow, then it would be easier to deepen the middle of the slots than the ends; we could use a short blade that fits in between the bindings. But we don't know that it has binding.
If there were a lot of chips from removing the frets, I'd rather fill those than sand them off. And what if the surface needed leveling? In both of those cases I would want to keep the same radius, but that's not a choice.
If the hypothetical player preferred one radius over the other, then I would change it to his preference; that means that either answer could be correct.
In either case we would have to reshape the nut, so that does not inform the decision.
Will the luthier see any difference in the two choices. Considering the information given, as an engineer or artist he would do it either way. As a manager he would advise against it unless he could charge extra.