Andy Dingley coughed up some electrons that declared:
Not forgetting duty cycle - the more "pro" ones will go longer without cutting out, but for DIY it's not so much of an issue.
I agree on the wire feed - nothing worse than a stuttery wire.
I think I used CO2 exclusively. I was so impressed compared to my abortive efforts with sticks that I was happy.
Mine was a low end Cebora - and it was *very* reliable. Nice wire feed speed control and current control gave repeatable results.
I do empathise with Dave though - rusty car thickness steel is probably the hardest. Landrover chassis is more pleasant and a bit of 1/8" angle is very satisfying.
Cheers
Tim