Powerisers are made in korea. Powerstriders are made in china. Powerisers have different springs, I think they are stiffer than the powerstriders. The powerisers have been having some problems with the frame warping but that might be fixed by now. I think powerstriders are probably a bit ahead of the powerisers atm. They now come with the upgraded bindings and calf cuffs and stuff. They are the same as projumps. Sprog will know more about this
Korea (but actually assembled in the phillipines) = HiTech & C, Co.
China = FlyingLocust/FlyJumper (Same company, seems to operate under 2 different names for some reason)
The Poweriser frame warping problem was fixed a while back Spud, they made a few changes to the construction and it sorted it out. Poweriser springs are fibreglass only, PowerStrider are a fibreglass/carbonfibre composite. Personally I'd say they're about equal in terms of build quality and lifetime. Other than that you're all cool Spud
AirTrekkers are the same as ProJump/PowerStrider/FlyingLocust/FlyJumper/etc/etc
AirTrekker as a company are regarded as a bit of a joke in the community as they attempted to rename the entire sport of PowerBocking to "AirTrekking" last year, under the argument "Skateboarding isn't named after the inventor - so why should this sport be either? Skateboarding you skate on a board, in this sport you Trek the Air - AIRTREKKING!". They pretty much realised their mistake after the massive community backlash against them
I can't remember anyone purchasing off them recently though, so I'd go with either of the two companies I linked to earlier.