I was speaking to a welder the other weekend on this issue, he uses a Gas Mig welder so i asked him what he thought. He said that he had seem some really shit welds from gassless Migs but also had seen some good ones. Gassless is good if your welding outside, if you use a gas mig outside the wind can disrupt the gas shroud thats means to form around the weld as you go.
Do not get a welder less than 150amp, otherwise it will not be man enough for the job. To sum it up gas is best