If your good with good technique and riding in just board shorts i.e. hot weather then, the waist is for you, if none of these, especially the bit about being good, then get the seat, you can hold a little more power with a seat and if your just beginning then the kite will spend alot of time above you at the zenith (pulling your harness up) so having a seat will help keep this from happening. I'm also told that what ever you choose first you tend to stick with in future, i ride a seat and have often thought of changing, the 2 are very different and it will take a while to adjust from one to the other.
Hope this helps
Regards
Neil