I hate when this happens.
We have 2 highly respected, highly educated, experts with a wealth of experience of details pertinent to a subject on which they have opposite opinions.
I've followed both Don Thomas and Ed Ashby for over 20 years and I put a great deal of credence in what they say.
In this case of which type of edge is better for a broadhead, I think I will lean toward Ashby's recommendations simply because of the incredible number of animals the he has killed with an arrow, and he studies the wounds in great detail.
I'm not discounting Don's opinions in any way, but Ashby's angle on this more mirrors (slightly) a hunting perspective.
My extremely small, anecdotal sample numbers has been that I've had better results with a mirror edge, than I had with a filed edge, but there are other factors to consider. My experience level is a bit higher these days than when I used a (shaving sharp) filed edge.
My opinion FWIW, If both edges are shaving sharp, they will do the job. When both edges are sharp, then more important is the quality of the head.