After carefully examining each spicy sandwich, I think the main difference lies in the sauce.
The piece of fried chicken in the Spicy Ch'King was glazed in sauce, while the Crispy Chicken fillet was topped with and perched on a large dollop of the stuff.
In my opinion, the glaze better allowed for even flavoring throughout each bite. With McDonald's dollop strategy, there were some bites of chicken that had no sauce at all in my sandwich — and therefore no spicy flavor.
Regarding flavor specifically, Burger King's glaze was more well-rounded. It offered notes of chili pepper without overwhelming my taste buds or taking away from the flavors of the actual chicken. I did think that a bit more salt would have added balance to highlight the fruitiness of the chilis.
While McDonald's sauce looked delicious, it was pepper-flake heavy and barely had any additional flavor. The mayo base brought a smooth texture and a slightly sweet flavor on first bite, but it quickly disappeared when a stinging heat took over.