I personally would not MIG 4130 since the nature of mig welding involves starting and stopping. but your right both are often done with success, I've seen Mig welded 4130 cages fail at the welds and almost kill the rally driver. so while it can be done. the average talent of the backyard car builder is not up to the demands. SCCA used to "require" ARC welding on all their rally cages. but "ARC" welding is a broad discription.

while the typical road racing cage will NEVER see the rollover stress of a rally cage, I prefer to use a more strigent standard when building a cage, since I've cross trained cars before, and IMNSHO being confident in a car is WAY more important then saving 100# on a cage. and while every "real" racecar engineer follows the rule of minilisim for a private funded team I think a good overbuilt cage will pay dividends in chassis longivity, and driver confidence.

Plus if you've ever had a problem with an event scruitneer you know how painful it can be to proove your legality. I like pulling the car up to inspection and hearing "WOW"