...pricing and weight have already been pointed out, but seems everyone skipped on the bio-mechanical and ergonomic aspects...
.....the primary difference between the two types is how much surface area is available for placing it against your shoulder pocket, should you choose to use it in that manner
for a solid mount. Both provide similar cheek weld, but when mounted to the shoulder, the SIG has more surface area available due the length/depth of the brace.
The SIG brace will reach from the top of the hollow of the shoulder pocket to the top of the pectoral muscle of the chest, just like a normal stock. This allows the user to raise the weapon high enough to achieve a good cheek weld without having to roll the face over to make contact with the stock/tube covering. The Thordsen has less surface area and forces the user to place the stock lower in the shoulder hollow to gain a "solid" mount point, but lowers the stock/tube and forces the user to roll the face over. Remember, everyone's physique differs so some may feel this to lesser or greater effect.
One way to imagine it is like some one pressing their palm into your shoulder area versus someone pressing their finger into the same area.....
Let functionality be your guiding criteria, not the flash or savings of a minimal amount of cash..... if it doesn't work for you it was a needless expenditure of money and time..