Ninja. Ninja. Just Ninja.
Ninja is faster, attacks faster.
I have found out that Axe's attack increases faster, but at lower levels their damage per shot is the same; however Ninja's faster attack rate compensates.
Ninja's health is slightly lower, but Axe's is only higher due to its higher max level. At lower levels Ninja Cat actually has more health, but Axe Cat's health has faster and greater growth...
Ninja > Axe.
In the game, on Ninja Cat's profile/cat description/biography, it is mentioned that Ninja Cat and Axe Cat are rivals. They are also the same cost, are both strong against Reds, and have the same damage per attack (for what I know). Their healths, I cannot be sure. Please post below, in this Cat Comparison debate, which one you think is better.