Originally posted by Mr.Alan maybe for horse race oder motorcross.
G'day and welcome to the forums.
Your choice of cameras for this style of photography is interesting as I thought they were both better suited to more static subjects. I use a K-1 and have captured motocross and Superbike racing and no doubt the D810 can do just as well if not better. I don't have experience with the D810 but I'd have thought there would be less difference in performance imagery than say the D5/D500 and the K-1 as these are cameras aimed at action photography.
Of course you may be like me and you're after a 'jack of all trades' camera as you don't have a limitless budget to double up on cameras for specific roles.
As suggested, if you can try renting this will give you a better understanding of what each can offer better than anyone's opinions.
Good luck with your decision, though to be honest these days there are no bad cameras, just cameras with features and compromises that will suit your needs better than other options.
Tas