[janalsncm]

Yes, it would be hard to avoid stepping into the scope of psychology to truly answer the question, because what might be easy for you might not be for me, even if we had equal elo ratings. (Trivial example, we might have equal elos but I try out a new opening you normally play.) You can probably learn a function to give a reasonable estimate though.

[mlyle]

I prefer something kind of objective rather than looking at player strengths. The "depth needed to search to pick equal move" is my best answer, maybe revised to be "and isn't found with a couple of simple heuristics" to clean it up some.