Some people can only use one or the other or they end up with a sore back.
Seat probably easier to learn with.
Different seat harnesses have different hook heights, some are about the same as a waisty.
I recently switched from seat to waist, what I notice is with the waist your legs are much more involved, on a seat you can just bend the knees and sit down to counter the force of the sail but with the waist you need to be pushing thru the feet then legs as you lean out. If you just sit with a waisty it will ride up to your underarms. I feel the waisty promotes better sailing stance and technique.
Also when sailing upwind on a waisty you can let the harness swing around your waist keeping the harness and hook pointed at the sail as you twist your body around to look and lean forward. I find this very helpful on the longboard.
And if it matters at all to you a seat harness looks like you are wearing a nappy