I, quite by accident as it happens, bought a dual purpose mig. This allows you to use the mig gasless if you buy the more expensive welding wire with the flux core. I really can't recommend this highly enough! Instead of the gas creating a "shield" around the welding reaction the flux core burns off and gives you the same shielding effect from the air allowing a good weld. The advantages are as follows:-

1) The amount of variables are cut down in the welder settings. You only have to worry about wire speed and voltage, gas regulation has been removed from the equation.

2) You can weld outside in a strong breeze as the gas shield isn't there to be blown away from the reaction.

3) Large and medium gas bottles can't be bought. You have to pay a yearly hire charge for them which isn't cost effective if you don't use the welder much.

4) Gas stored under pressure can be dangerous.

5) Your welder becomes more portable without the bottle and it's stand.
Here's the single greatest piece of knowledge involved in mig welding.Push the weld, don't draw it as with other types of welding.(if you hold the torch in your right hand you will be moving from right to left).