The traditional view is that seat harnesses are for racing; and waist harnesses are for freestyle, wave and freeride sailing. By and large this is also the current view, but ultimately it is down to personal choice.
A seat hugs your backside and has straps around the upper legs to help keep it there. The low hook help control power.
Waist harnesses go around the lower torso area, and because of their higher hook positions encourage a stance that is more upright or 'on the toes'. This make them the preferred choice for more 'active' sailing disciplines - such as freestyle or wavesailing - where ease and speed of hooking-in / unhooking is very helpful.
Most waist harnesses nowadays are made from pre-thermoformed foam materials that do not absorb water. The thermoformed material is also relatively resistant to sliding when compared to older 'classic' harnesses that were mostly made from polyester based materials.
Classic build harnesses constructed from polyester have a price advantage of around 30% and - when dry - may be lighter than thermoform harnesses.
The support felt will depend upon where it is directed and the size of the area supported. Some harnesses have plastic reinforcement plates to spread the support over a wider area, other harnesses have less support to allow for more unrestricted upper body movement.
Hook height is largely a matter of personal preference.
Seat harnesses designed for racing have the lowest hook heights, allowing for very free upper body movement and the ability to hold down lots of power. Freeride seat harnesses have a medium height hook that allows for more back support.
Waist harnesses have the highest hook positions, and are the most popular choice with sailors who are not dedicated racers. The high hook gives a very direct feel and allows for easy power-on / power-off transitions - meaning that waist harnesses are well suited to manoeuvre-orientated sailing.
Note that waist harnesses, if strapped too tight around your middle, can impede easy breathing. However, a snugly fitting harness - that is not over-tight - will also do a good job of keeping your lower back and kidneys warm in winter.
A perhaps useful feature on some harnesses - though not necessarily for keys with electronics inside - though good for a spare length of rope in any case!