A belt that keeps you in the seat is the better design, rather then merely attached to the floor, IMO. The theory being, if the seat moves, and the belt does not, the belt then becomes loose.
That sounds reasonable, but more likely I (and the seat) will be moving forwards in a crash as the front of the car crumples in deceleration, so the belt will become tighter...
It doesn't do any good to stay strapped in the seat if the seat doesn't stay attached to the floor.
