All of the mentioned machines are AC generator drives. They all make excellent generators and decent welders... They are all very consistent for structural work. If you want a really good pipe only arc look at something else. I have always felt a lot of people choose these on loyalty of color. But I tend to look at the engine performance in this category. The Lincoln I own welds excellent for its given output. One draw back I have noticed compared to some Millers is gas consumption. Mine is fairly hungry even when tuned up and running perfect. All the new Lincolns carry a different identification system BTW.
As far as arc characteristics I like my Lincoln hands down for 6010 work and in general. The 7018 side seems very comparable. I am partial to RED they just weld better to me even though everything "inside" my shop is blue.
My Ranger 10 is a good size for everything I do except Air Arc Gouging.....It only runs a 3/16 carbon without over working the machine. Its rated around 270 amps so it pulls it hard.
My suggestion is buy the biggest unit you can afford if you wanna run Carbons.
Another wish is full on diesel powered unit and the built in Air Pack.....IE an equipped Trailblazer with diesel. But I don't think that combination is available. AND LINCOLN DOESN'T DO THIS UNTIL ITS FULL BLOWN HEAVY COMMERCIAL ORIENTED
Lincoln this bites.
SHORT ANSWER:
Structural work, hand rails and light repair they all work. Heavy equipment, railroad, mining......go to a much bigger platform. Pipe don't waste your money, much better stuff available.