Depending on the age of your children, I think that they will love doing both. My children are 11, 8, and 6, and they really enjoyed both pin trading and getting autographs. The autograph books are fairly cheap but the pins can get costly. I have to say that on our first trip, we did not pin trade so my children really enjoyed the autographs, but on our last trip in mid-December, we did pin trade. It was one of the highlights of our trip. My oldest two were always on a mission for certain pins while my youngest just looked for his favorite characters and rides. All of the cast members were extremely kind and patient. Now, we can't imagine going to WDW without pin trading. I would have to say that at this point my children would choose pin trading over autographs.
Have fun!!