We started off my two boys on a keyboard with weighted keys, before we committed to a real piano. I always knew no matter how good the keyboard is, it is still not a piano. However, we didn't want to invest in a piano until we knew the boys would continue the piano lessons.
I would recommend to get a used keyboard with weighted keys first. If your DS is really into the instrument, you'll need to get a piano within 6 to 12 months. That's what our piano teacher recommended and that's what we did (we got a piano just over a year after they started the private lessons with the teacher).
We now have both the keyboard and the piano. My boys often go to the keyboard to play something fun because it has many different options and features. So it was not a total waste of money :-).
HTH
