The Soul is not made in the normal Flying Wing factory. It is made by a select group of fliers who also make and fly the Prediction.
The Silver Fox line is made in the regular kite factory, which makes authorized kites for many other kite companies.
The Silver Fox line is made to a price point (lower quality fabrics) and some of the older ones out there are showing signs of age. The Soul has a polyester sail (not sure if it is real icarex) and is of much better quality. The frames on both are Dynamic rods. The Prism E2 probably sits between the two in terms of quality