Arc welding does not require the pristine clean joint that mig does and it has less chance of inclusions in the weld.
Tig is the first choice followed by arc then lastly by mig in my opinion.
Not sure I follow your logic. One minute you praise stick welding as it "does not require the pristine clean joint that mig does", yet state in the next sentence that TIG would be your first choice. TIG requires just as clean a surface, usually even more so than MIG, so I am not sure what you are suggesting. Plus, it is considerably a slower process, and certainly would not be my first choice for a cage.
Just a little food for thought, but the majority of NASCAR cages (if not all) are MIG welded....not stick or TIG.
Never seen mig rated as an acceptable method of welding structural joints. But I don't hold a cert in structural welding or anything.
Its my understanding, and I could be completely wrong here, that the reason you see stick primarily in structural welding is twofold. One, its a lot easier to run longer leads, and two, the flux coating shields better in an outside environment than MIG shielding gas. It would be difficult to try and feed MIG wire through 50-100' of liner, whereas running 100' of stick leads is a piece of cake. Also, have you ever tried MIG welding outside? It can be a nightmare with even the softest of breezes, which isn't much of an issue with the flux on a stick electrode.
As far as the topic at hand is concerned, there is no way you can get a structurally better weld with stick over MIG on a roll cage. 7018 is still 70k rod, and ER70s is rated the same. Only issue one would have is if the amperage wasn't comparable....say a 140a MIG vs an AC/DC 200+ amp stick.