If you do wing chun get a wing chun dummy.
People say that material does not make a difference and the placement of the arms doesnt make a difference.
What I know is this:
1. The material determines what noise is made when you strike it, the best material that reflects the energy of the hit is hardwood (oakwood is what mine is made from)
2. How it is held in place also makes a difference. The free standing ones do not really allow the wooden dummy to vibrate back and forth and therefore IMO there is no point in doing the pulling techniques (lap sau and mengin sau (sp) etc) since you will not feel the correct amount of leeway and you wont be able to time your follow up correctly. The dummies that have the horizontal frames are the best for that.
3. Some people argue that the position of the arms dont make a difference. I WC they make a huge difference, when you do the wooden dummy form you will realise that transition and positioning is what it is all about. I think WC is generally about positioning. You dont learn how to block specific strikes you learn how to protect your gates, and that is why positioning in WC is probably more strict than other MAs. The arms do not represent actual arms. They are there to split the body of the opponent into zones. The arms can represent a hook or a jab or a roundhouse kick or an elbow or anything alse that you can think of.
Last but not least, ask your sifu about it!