The advantages of having a waist harness is that you will have much better flexibility and range of movement and of course it is more comfortable. It will cause you to develop your core strength more and because of this they can cause strain to the lower back. The big downside for a beginner is that they will ride up your chest more easily.
The seat harness will not ride up and is kept down by the leg straps. It also allows the rider to edge more easily with less physical effort. It is actually easier to do rotational tricks with the seat harness as it pivots just below the centre of gravity but it won’t look so cool!
It is advisable as a beginner to use the seat harness for lessons until you are up and riding but due to it being restrictive around your leg you will more than likely switch to a waist harness very soon.