Very simplistic, the shading and especially the eyes are fantastic ^_^ Though, it seems to me, that the ear is just the tiniest bit too thin. Her left eye seems a bit small as well, but it gives her a feeling of slight insanity and even balances out the image strangely. Overall, a very interesting piece.
Either would work honestly. Pinkie Pie would totally make the perfect "jack-in-the-box" kind of toy; a different surprise face/reaction every time. And Derpy, well if she were in the same concept, say she has a delay before coming out, it would be ann official derp moment, making it a fun derp toy. Then I could also make the joke of "Welcome to Derp in the Box! May I take your order?" XD, so yeah both would work.