I may say Sleepy-chan is more easy to use.
She's fast, she has 1.2 activation.
And still dev will buff her atk buff a bit in future.

On the other hand, Sheep-chan Pygmy... something is a lot slower than sleepy-chan.
Also she doesn't have 1.2 activation, that's mean you need some activation boost for her.
And if you need her group to reach the destination first, speedy unit also need.