I personally dont like to do TIG welding with carbon steel fillers. Porosity problems, some of which you cant see. I can do it I just dont like to. If it were me using TIG, I to would choose the 312 SS. I have done 4130 with MIG using er80s6 and beat, twisted, and bent the heck out of it with no weld zone problems at all. ER70S6, 80S6 and 312SS all work well with both metals and are all tougher than mild steel. I have never had it subjected to the type of stress encountered during a racing crash.
I was involved as a judge in a Mini Baha put on by engineering schools three years ago. If you ever get the chance to be a judge, take it. It's genuine hoot. Any how, They require 4130 tubing cages properly braced with at least, good looking welds (bird poop not allowed). We had several rollovers, end over end, and sideways, and with 106 cars entered, and at least 106 diffrent non professional welders, no cages let loose. All the students I questioned used er70s6, and used both TIG and MIG. Granted these only weigh a couple of hundred pounds and run 10 HP engines, but they really took a beating.
Now you know what I know (which aint much). Hope this helps