CoachK:

Your best bet is to go to http://www.lincolnelectric.com/products/litrequest/
and download the manuals, but basically, the 175T has a stepped transformer voltage switch, the 175 Plus has a continuously variable voltage switch and the 175 Weldpak is Lincoln's hardware chain brand (same as the 175T, but often sold as a wire feeder without mig capability, or with an add-on kit to get mig capability, and not sold by welding equipment dealers). Some say the Weldpak
equipment also has cheaper wirefeed and other components. The continuosly variable switch allows for finer adjustment and can be changed while welding.