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They will weld and do a job but the difference between gas and gasless is a much nicer weld and probably easier to weld, especially on the thinner materials.
You could always buy a dual purpose machine but for more accurate advice on here you need to provide more info...budget, what you intend to weld, how often, indoors or out etc
Hi I'm on my second welder a clarke 151en , the first was a cosmo (sip) which i only used gasless, i tried the gasless on the clarke it was ok but on gas it's so much better i don't think i'd go back to gasless now. I'm still very much a novice but using gas makes it easier to get a reasonable result.