Unlike bowling you do not get to bring your own ball to the game. So it really does not matter what she likes. Get her what she will be throwing in games.
I agree and will add on to that. I have a bucket of assorted balls and randomly pick out 2-3 of them for a practice session. Every time I throw a different ball back to my DD, she has to pitch with whatever she gets. I'm hoping that in this way she doesn't get it in her head that her pitching depends on the ball and learns to deal with whatever she has to work with.