I’m surprised. My guess would be that if you are interested in shadows to a high degree of accuracy you must be in architecture or a closely related field. That would imply to me that you must have a working relationship with some surveyors who could probably give you true north with +/- 1 degree or better from any spot you pick.
At any rate, before you get into Rhino/Revit argument with a client I’d think you’d want to establish a method to determine which one is closer to reality. And I agree with Joerg.H that 2-3 degrees difference, even from reality, just doesn’t make any practical difference.
And either way, whichever one is found to be the more accurate, I’m sure the other guy would appreciate the info so they can tune up their software.