Krita is a painting application that supports classic hand-drawn animation, while adobe After Effects is a compositing suite, so they are quite different. Krita might have a little simpler learning curve, but you need to already be good at drawing and know the craft of classic cartoon, while After Effects can be used to make stop-motion like animations.
Both programs are professional tools so they have tons of features. I have used Krita for a long time and still find new useful things hidden in the interface.