Hatsan torpedo 155 vs torpedo 150

Within both models there are two variants. Vortex and Vortek. One uses a gas spring, the second uses a mechanical spring.

Other than that critical variation, the 150/155 are largely the same, other than the Turkish Walnut stock (and weight). 

It is worth noting that Hatsan tends to put out realistic values for their FPS, using regular lead pellets. This is very different from other companies, which use optimistic values, which assumes some kind of super-light alloy pellet.