Neither of course. it is better to have a hook surgically implanted into your gut. That way you never forget to bring it to the water and have to go home.
Seriously that question is like asking which is better Coca Cola or Pepsi. The answers will always vary depending on windsurfing style, comfort, and technique.
Typically speed and racers use seat harnesses, while wave and freestylers use waist harnesses. But for every generalisation there is an exception, and there are many racers that prefer waist harnesses, and vice versa (waist harnesses that prefer racers!!!!!!!! ok maybe not).
It is best to try both and find out for yourself which suits you, your style and discipline best.
JB
Seat harnesses are more popular and more effective for bigger sails and flatter water. Almost all speed sailors use them, hardly any wave or freestyle sailors use them and among slalom sailors there is a mix.
In places that are very windy and rough such as Maui, The Canary Islands, or aff the coast of WA, most slalom sailors will use a waist, but when the water is flatter more will tend to use a seat. Some sailors (including Antoine Albeau) will use either, depending on the conditions.
The advantage of a seat is that the lower centre of gravity makes it more powerful, meaning it will take more of the weight off your arms and enable you to more easily control a bigger sail. This comes at the expense of ease of hooking and un-hooking, and also reduces the mobility of the lower half of your body.
The advantage of the waist harness is that your legs have full mobility to absorb chop in rough water conditions when it's more important to have control of the board than the sail.
I use a waist harness, but would consider changing to a seat for sails 6.6m and up, on flat water, but I'm not sure I want to be changing from one to the other.
I find a waist harness far more beneficial in catapulting me even further given the higher position
With the average age of windsurfers steadily increasing the popularity of the seat harness is being overtaken by the wheel-chair harness
Waist harness: people on the beach can still see and admire your backside, so all those hours on the stairmaster aren't wasted 
Seat harness: looks like a big nappy and people can't see your bottom. Popular with speed sailors and from what I have seen it seems the speed sailors are kind of on the large side so perhaps they only like to wear a seat harness to cover up their backsides.
You bunch of girls still using harnesses. I don't even bother. Just muscle up pussies.
