I learnt and used a seat harness for years, left and came back to the sport this season, and started with a NP racing seat but when I upgraded all my gear, i figured to upgrade my harness also, I was skeptical about the waist harness at first after sailing in a seat for 8 years and the first gen of waist harnesses were not the best in terms of back support.
BUT NOW.... things have changed and I wouldn't go back to my seat harness. My stance, support and ease of hooking in/out is improved (as hook tends to be a little higher positioned), but it did take time to adjust from seat to waist at first - 2-3 sessions.
One issue I did find with the selection of waists harness' is the kidney belt strap, the first few i tried had a poor overlap and grip and thus I couldn't get the locked in feel around my waist. Also a bar release makes jumping in and out quicker but isn't essential. I ended up with the the Pro-Limit Team Wave Waist and give it 3 shinny teeth..

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If your on flat water and thinking of mainly slalom/flat water bump and jump, then a seat harness would be fine but wavesailing/ or thinking of getting into it at any stage, a seat does limit your ability to progress, as hooking in/out becomes a frequent thing, and with the stance that you end up in when transitioning between moves, the waist harness comes up on top.... that being said, Robby Naish can still be seen wearing a seat harness every now and then in the maui surf....
Get into your local store and try as many on and lean off the rope they usually have attached to a wall somewhere to see how the support feels in your back and when you bring it home try it on again over the top of waterever you sail in to be sure it fits a sits right. Not sure on your budget but you'll probably need to spend about $200 on one and it will be worth it for a harness as it will become the most used piece of equipment you have, the entry price ones at $140ish seem to have the ergonomic problems with them I mentioned earlier.