DerikL: I think vipermanz is correct. I did a good bit of research before I bought my lincoln sp175. The only difference I found is the sp175 PLUS has infinite control over arc voltage, and the others have 4 or 5 set points.
I spent the extra to get the PLUS, but so far, have not found it necessary. It might come in handy if you are trying to weld thin material and need that extra control over the arc.
Hey! I'm as old as dirt!... But the grass still likes it!
Don't forget our troops!