I actually prefer KATO over Athearn, and find the Kato not only runs better, but the detailing is better as well.
An important (and deciding) factor for me, when I was comparing them side-by-side in the hobby shop, was the black caulking that surrounds the front windows. KATO has a nice, tubular piece surrounding the windows (as per prototype), while Athearn has a flat (and too wide) piece that looks like it's painted behind the windows.
KATO also comes with windshield wipers and a variety of other details, although if you add your own detail parts I assume that's not an issue.
There's just something about Athearn's SD70M that looks "toy-ish" to me, but that is just my opinion. KATO would be my recommendation.