Waist ,,,, In general means more lower body freedom for wave sailing and freestyle.
Seat ,,,, In general means more leverage and pressure for speed.
However, the most important thing to consider is your own personal stance/riding style.
Along with boom height and harness line length, stance will determine which of the two harness options will be most efficient/effective.
Note,,,, Ross Williams Top 3 in the Formula world champs used a waist harness.
,,,,,,,,,, Sam Parker Top 3 in Australia formula/Slalom,,,, etc,,,, uses a waist harness.
Interesting note,,,,,,, Some racers use a waist harness to world class level, but I haven't heard of any wave or freestyle sailors using a seat harness.