As far as I know, Airborne is still the importer. The difference in retracts is a lot of personal preference but there is difference.
Spring air comes down when air is lost, Robart and Century do not. Some people like this, I don't because the retracts are always down when transporting and this can be a problem.
Robart and Century have robo struts for their retracts, I don't know about Spring air.
All three are air but Robart has mechanical also.
Send me a pm for more info.